Leaching of non-treated ilmenite by HCl-CH 3 OH-H 2 O mixture and its kinetics

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1 Indian Jurnal f Chemical Technlgy Vl. 3, January 2006, pp Leaching f nn-treated ilmenite by HCl-CH 3 OH-H 2 O mixture and its kinetics M A Habib *, R K Biswas, M R Ali & A K M Hasan Department f Applied Chemistry and Chemical Technlgy, Rajshahi University, Rajshahi 6205, Bangladesh drhabib6@yah.cm Received December 2004; revised received 2 July 2005; accepted 0 September 2005 The effect f the additin f methanl t leachant hydrchlric acid n the disslutin f nn-treated ilmenite has been investigated. The kinetics f the disslutin prcesses have als been investigated. The disslutin f ilmenite is enhanced by the additin f little amunt f methanl in hydrchlric acid. The leaching with 6 M HCl -0.5 M CH 3 OH-H 2 O mixture at 0 C and slid t liquid rati (S/L) f 0.02 g/ml results in the disslutin f 9% Ti and 95% Fe cmpared t disslutins f 53.5% Ti and 60% Fe in 6 M HCl alne. Decrease in leaching temperature and HCl cncentratin and increase in S/L rati result in decrease in the extent f disslutin percentages. The kinetics f disslutin have been investigated by measuring the dependences f rate f disslutin n HCl cncentratin and temperature f leaching. The values f activatin energy have been evaluated frm the temperature dependence rate data. The kinetic data culd be fitted t the Shrinking Cre mdel f disslutin. Keywrds: Leaching, Kinetics, Ilmenite, HCl-CH 3 OH-H 2 O mixture, Disslutin IPC Cde: Int. Cl. 7 B0D2/00 A cnsiderable amunt f heavy mineral depsits cntaining ilmenite has been discvered in the sutheastern castal area f Bangladesh. It is reprted,2 that the castal sand cntains mre than 0% heavy minerals, which in turn cntains abut 27% ilmenite. Several attempts have been made t develp a suitable prcess t recver titanium frm ilmenite sand. Ti(IV) has been separated effectively frm Fe(III) present in a sulphuric acid leach liqur using slvent extractin, stripping and precipitatin techniques 3-6. The leach liqur was btained by heating the pwdered ilmenite sand in 8 M H 2 SO 4 fr lng perids, usually >48 h. Past experience suggests that leaching with dilute mineral acid, such as HCl, HNO 3, H 2 SO 4 and HClO 4, is nt at all effective 7 fr the disslutin f ilmenite. Only hydrfluric acid was fund t be effective 7,8. Hwever, this methd suffers frm sme disadvantages such as the difficulties arising during handling f hydrfluric acid, high leaching time and invlvement f many steps t recver the desired prduct 9. Recently, several authrs have studied the leaching f ilmenite by HCl (refs 0-5), H 2 SO 4 (refs 6, 7) and thers 8,9. Jana et al. 20 and Yrukglu and Girgin 2 used HCl-CH 3 OH-H 2 O slutin fr leaching f deep sea ndules and La 2 O 3, respectively. Althugh, alchl alne has n ability t leach the ilmenite, mixed leaching agents such as alchl and mineral acids have been selected with an aim t imprve the leaching quality f ilmenite t a leached prduct. Previusly 22, it has been reprted that hydrchlric acid-alchl (ethanl) mixture shws a significant effect n the leaching f ilmenite. In anther paper 23, the leaching f nn-treated ilmenite by HCl-C 2 H 5 OH-H 2 O and HCl-C 6 H 5 OH-H 2 O mixtures and their kinetics have been reprted. This paper discusses the leaching f ilmenite by HCl- CH 3 OH-H 2 O mixture. The kinetics f the leaching prcess have als been investigated and discussed. Experimental Prcedure Reagents Ilmenite sand was cllected frm the pilt plant f the Bangladesh Atmic Energy Cmmissin (BAEC) at Cx s Bazar (abut 60 km suth f the prt city f Chittagng). Accrding t BAEC, the cmpsitin f the received sample was: TiO 2, 38.5; Fe 2 O 3, 25.62; FeO, 29.75; SiO 2,.2; P 2 O 5, 0.03; MnO 2,.3 and Cr 2 O 3,.08%. The chemicals f the fllwing specificatins were used in this investigatin: TiO 2 (E. Merck, 99%), FeNH 4 (SO 4 ) 2.2H 2 O (E. Merck, 99%), NH 4 SCN (BDH, 98%), H 3 PO 4 (BDH, 85%), HCl (BDH, 36%), HNO 3 (BDH, 69-72%), H 2 SO 4 (BDH, 98%), H 2 O 2 (BDH, 30%), CH 3 OH (BDH, 95%).

2 54 INDIAN J. CHEM. TECHNOL., JANUARY 2006 Methd The sand was dry-grund and sieved t cllect particles f >63 μm-<25 μm in size. A knwn amunt f sand (usually g) was heated in a pyrex flux under reflux with a knwn vlume f leaching agent (usually 50 ml) at a predetermined temperature. A magnetic ht plate was used fr heating and agitatin (300 rpm). At intervals, duplicate aliquts (0.5 ml) f slutin were remved, diluted and analyzed clrimetrically; ne fr titanium cntent 24 and the ther fr irn cntent after xidising with nitric acid 25. In stage-wise leaching, the residue was filtered ff and heated with 50 ml fresh leachant. T study the effect f slid t liquid phase rati n leaching, the amunt f ilmenite was kept cnstant and the vlume f leaching agent was varied. A plastic cated stirrer bar was used fr magnetic stirring. During temperature dependence study, the leaching flask was placed inside a thermstated water bath. Theretical analysis At a cnstant temperature, phase mixing rate and slid t liquid phase rati, the rate f disslutin f a slid by a liquid reagent may be expressed as 26 : d W = k.a.c a () dt where W is the weight f the unreacted slid at a time t, A its surface area, C the reagent cncentratin, a the reactin rder with respect t the reagent cncentratin and k is the rate cnstant. If W is expressed in g, A in dm 2, C in ml dm -3 and t in s, then k will be in g dm s - ml - prvided a is unity. In rder t get an integrated frm f rate equatin frm Eq. (), the variatin f A with t must be taken int cnsideratin. This requires the cnsideratin f using unifrm spherical particles in the experiment. If a very clsely sieved fractin is used in experiment, then a definite amunt f pwder will cntain apprximately the same number f particles. Let g sample f particle size between 63 and 25 μm cntains n number f particles n an average, then, the area ccupied by g pwder (A), is apprximately given by: A = n 4 π r 2 (2) where, r is the radius f each particle. As the radius f each particle (r) decreases with time f leaching, the weight at any instant can be calculated frm the fllwing relatin: W = n (4/3)π r 3 ρ (3) where, ρ is the density f the material. Frm Eqs (-3), the fllwing equatin is btained: d r dt = a kc ρ (4) Nw n integratin within the limits r = r when t = 0 and r = r when t = t, Eq. (4) gives (C is assumed cnstant in this interval f time): r R = r = kc a t ρ The fractin reacted, R at any time is defined as: W W W (5) (6) where W and W are the initial weight and weight after certain time, respectively Then fr spherical particles, R may be presented as fllws: 3 3 (4/3) π r ρ (4/3) π r ρ R = 3 (4/3) π r ρ Eq. (7) gives: ( ) 3 (7) r = r R (8) With the help f Eq. (8), the Eq. (5) takes the frm: ( ) /3 a r / r R = kc t ρ r, ( R) /3 = qt (9) a where, q = pseud rate cnstant = kc / ρ r. It is imprtant t nte here that the final rate equatin is independent f number f particles but dependent n the radius f each particle n which the rate is varied inversely. Equatin (9) is based n the Shrinking Cre mdel f particles during leaching. Equatin (9) states that the plt f ( R) 3 versus t at a certain reagent cncentratin yields straight line with slpe equalling t q. Fr a pwder f clsely related mesh number, the terms ρ and r in q are cnstant; s that the value f q will vary apparently n the reagent cncentratin, C. If the values f q HCl are determined frm the slpes f ( R) 3 versus t plts at varius reagent cncentratins (HCl), then the fllwing relatinship will hld:

3 HABIB et al.: LEACHING OF NON-TREATED ILMENITE BY HCl-CH 3 OH-H 2 O MIXTURE AND ITS KINETICS 55 lg q = lg ( k/ρ r ) + a lg [HCl] (0) HCl Equatin (0) indicates that the plt f lg q HCl versus lg[hcl] will give straight lines with slpe equalling t a and the intercept (I) equalling t lg ( k ρ r ). It is, therefre, apparent that the value f the disslutin rate cnstant, k can be evaluated frm the value f I prvided ρ and r are knwn fr the pwder under investigatin. T make a study n the effect f temperature n the rate f disslutin, q T values are t be determined at different temperatures, T (K) and the data shuld be pltted accrding t the Arrhenius equatin: lg qt = cnstant ( Ea/2.303 RT) () where, the symbls have their usual significances. The slpe f the plt f lgq T versus ( T ) is equal t Ea 2.303R, s that the value f the apparent activatin energy, E a culd be easily calculated. Results and Discussin One gram nn-treated ilmenite sand having particle size f μm was leached with 50 ml f mineral acids (HCl, HNO 3 and H 2 SO 4 ) and methyl alchl (0.5 M) mixtures at their biling temperatures. Fig. shws the effect f leaching time n the weight percentages f titanium and irn disslved. It is fund that the wt% metal disslved (percentages f titanium and irn disslved frm the sand) is increased with increasing the leaching time up t abut 7 h. Within this time abut 9% Ti and 95% Fe are disslved in 6 M HCl-0.5 M CH 3 OH-H 2 O mixture. Under similar cnditin ~34% Ti and 47% Fe, and 0% Ti and 25% Fe are disslved in 3 M H 2 SO M CH 3 OH-H 2 O and 6 M HNO M CH 3 OH-H 2 O mixtures, respectively. It is reprted earlier 27, that, the leaching f nn-treated ilmenite by 6 M HCl alne yields 53.5% Ti and 60% Fe disslutins. Therefre, it is cncluded that a mixture f 6 M HCl-0.5 M CH 3 OH- H 2 O is a better leaching agent than 6 M HCl alne, 3 M H 2 SO M CH 3 OH-H 2 O and 6 M HNO M CH 3 OH-H 2 O mixtures fr leaching f nn-treated ilmenite sand. It is cncluded that increased leaching in HCl mixed with alchl might be due t a certain change in the interfacial prperty f ilmenite/leachant interface. Since alchl is less plar than HCl and H 2 O, it pssibly penetrates the pres f ilmenite. This wetting facilitates the diffusin f HCl in the infrastructure f ilmenite and cnsequently, enhanced leaching is bserved. The effect f HCl cncentratins in the leachant cntaining 0.5 M CH 3 OH n the maximum wt% f Ti and Fe disslved frm ilmenite is shwn in Fig. 2. It indicates that the disslutin f the maximum wt% f Ti and Fe increases extensively with increasing HCl cncentratin in the leachant. Hwever, ver 6 M HCl cncentratin, the variatin in the extent f leaching is almst negligible and hence 6 M HCl mixed with CH 3 OH is suggested t be used as a leachant fr nntreated ilmenite. The effect f methyl alchl cncentratin in 6 M HCl n the disslutin f ilmenite is shwn in Fig. 3. Fig. Effect f leaching time n the disslutin f metals frm nn-treated ilmenite by different acid alchl-water mixture. Leachant cmpsitin: ( ), 6 M HCl-0.5 M CH 3 OH-H 2 O; ( ), 3.0 M H 2 SO M CH 3 OH-H 2 O; ( ), 6 M HNO M CH 3 OH-H 2 O. S/L = 0.02 g/ml; temp. = 0 C; stirring speed = 300 rpm. Slid symbls are fr Ti and pen fr Fe. Fig. 2 Effect f HCl cncentratins in the leachant n the maximum wt% f Ti and Fe disslved frm ilmenite. [CH 3 OH] = 0.50 M; S/L = 0.02 g/ml; temp. = 0 C; leaching time = 6 h; stirring speed = 300 rpm.

4 56 INDIAN J. CHEM. TECHNOL., JANUARY 2006 Fig. 3 Effect f methyl alchl cncentratin in 6 M HCl n the disslutin f ilmenite. S/L = 0.02 g/ml; temp. = 0 C; leaching time = 6 h; stirring speed = 300 rpm. It is seen frm Fig. 3 that the maximum disslutin weight percentages f bth Ti and Fe increase with increasing methanl cncentratin in the leachant up t 0.5 M. On further increasing the methanl cncentratin in the leachant, the maximum disslutin wt% f bth Ti and Fe decrease. Therefre, it is cncluded that the effective cncentratin f methanl in the leachant is 0.5 M. The effect f temperature n leaching f nntreated ilmenite by 6 M HCl-0.5 M CH 3 OH-H 2 O mixture is shwn in Fig. 4. Figure 4 indicates that the disslutin percentage f titanium and irn increases extensively with increasing leachant temperature. At 45 C, the disslutin percentages f Ti and Fe are 9.6 and 2%, respectively. Hwever, at 0 C, these values are 9 and 95% fr Ti and Fe, respectively. Frm the abve study, it is cncluded that temperature has a significant effect n the leaching f nn-treated ilmenite by HCl-CH 3 OH-H 2 O mixture. The effect f slid t liquid phase rati n the disslutin f ilmenite by HCl-CH 3 OH-H 2 O mixture has been investigated. Figure 5 shws the percentage f metals disslved n leaching at varius slid t liquid (S/L) phase ratis (g/ml). In these experiments, L is always taken as 50 ml f 6 M HCl-0.5 M CH 3 OH-H 2 O mixture and S is varied frm t 4 g ilmenite. It is bserved frm figure that the leachable percentages f bth Ti and Fe decrease cnsiderably n increasing the S/L rati. At S/L rati f 0.02 g/ml, the disslutin f titanium and irn are 9 and 95%, respectively; and the respective values decreases t nly abut 37 and 53% at S/L rati f 0.08 g/ml. This result shws that the leaching f the nn-treated ilmenite sand by HCl-CH 3 OH-H 2 O mixture is very much dependent n S/L rati. Figure 6 represents the effects f the slid t liquid phase rati n the cncentratins f Ti and Fe in Fig. 4 Effect f temperature n the leaching f nn-treated ilmenite by 6 M HCl-0.5 M CH 3 OH-H 2 O mixture. S/L = 0.02 g/ml; temp. = 0 C; leaching time = 6 h; stirring speed = 300 rpm. Fig. 5 Effect f slid t liquid phase rati (S/L) n the disslutin f ilmenite by 6 M HCl-0.5 M CH 3 OH-H 2 O mixture. Leaching time = 6 h; stirring speed = 300 rpm; temp. = 0 C. Fig. 6 Effect f slid t liquid phase rati n the cncentratin f Ti and Fe in the leached slutin. Leaching agent, 6 M HCl-0.5 M CH 3 OH-H 2 O mixture; time = 6 h; stirring speed = 300 rpm and temp. = 0 C.

5 HABIB et al.: LEACHING OF NON-TREATED ILMENITE BY HCl-CH 3 OH-H 2 O MIXTURE AND ITS KINETICS 57 leached slutin. It is seen frm Fig. 6 that the cncentratin f bth metals in the leached slutin increases with increasing S/L rati in the system. Hwever, the increase f titanium cncentratin in the leached slutin with the increase f S/L rati is very lw as cmpared with the cncentratin f irn. At higher S/L rati, Fe is preferentially disslved in HCl medium, whereas, TiO 2+ is thermhydrlysed t frm insluble H 2 TiO 3 (metatitanic acid) accrding t the fllwing reactin: TiO H 2 O H 2 TiO 3 + 2H + (2) which happens pssibly when the aqueus titanium cncentratin reaches t the value f abut 6 g/l. This result indicates that the leaching at higher slid t liquid rati cannt be suggested t get higher cncentratin f Ti and Fe in the leachant. Hwever, the stage-wise leaching is recmmended fr higher recvery percentages f Ti and Fe in the higher S/L rati. The kinetics f the disslutins f nn-treated ilmenite by HCl-CH 3 OH-H 2 O mixture has been investigated at varius acid cncentratins and the data are fund t be fitted by the Shrinking Cre mdel f leaching. Fr the Shrinking Cre mdel, the initial disslutin data give ( R) 3 versus t plts as straight lines, where R represents the rati f the amunt f titanium r irn leached t the amunt f titanium r irn present initially in the sample, respectively. Fig. 7 shws the typical ( R) 3 versus t plts fr disslutins f bth Ti and Fe by HCl-0.5 M CH 3 OH-H 2 O mixture at its biling temperature and at S/L rati f 0.02 g/ml. Since straight lines are btained fr bth metals, the slpe f the lines will be equal t q, the pseud rate cnstant in Eq. (9). The values f q HCl, min - at varius hydrchlric acid cncentratins have been calculated frm the slpe f the lines. The calculated lgq HCl values fr the disslutin f bth the metals at varius acid cncentratins are pltted in Fig. 8 as lgq HCl versus lg[hcl] plts at cnstant CH 3 OH cncentratin f 0.5 M, biling temperature and S/L rati f 0.02 g/ml. In bth cases straight lines are btained. Slpes f the plts fr Ti and Fe are 2.0 and 2.5, respectively. It is, therefre, cncluded that the expnent a n the reagent cncentratin fr the disslutin f Ti and Fe frm ilmenite are 2 and 2.5, respectively. Intercepts f the plts fr Ti and Fe are 3.95 and 3.90, respectively. Fig. 7 Typical [ ( R) /3 ] versus time plts fr the disslutins f Fe and Ti by HCl-0.50 M CH 3 OH mixture at biling temperature and S/L f 0.02 g/ml. [HCl]: ( ), 6 M HCl; ( ), 5 M HCl; (, ), 4 M HCl and ( ), 3 M HCl. Slid symbls are fr Ti and pen fr Fe; stirring speed = 300 rpm. Fig. 8 lgq HCl versus lg[hcl] plts fr the disslutins f Fe and Ti by HCl-0.50 M CH 3 OH mixture at biling temperature, S/L f 0.02 g/ml and stirring speed = 300 rpm. Temperature dependences f rates f titanium and irn disslutins frm ilmenite by 6 M HCl-0.5 M CH 3 OH H 2 O mixture are given in Fig. 0, where lg q T has been pltted against 0 3/ T(K - ). The values f q T, min - at varius temperature have been calculated frm the slpe f the lines f [ ( R) 3 ] versus t in min plts (Fig. 9) fr the disslutin f bth Ti and Fe by 6 M HCl-0.5 M CH 3 OH-H 2 O mixture at varius temperatures and at S/L rati f 0.02 g/ml. The experimental pints in bth cases in Fig. 0 d nt fall n straight lines. Curves are btained with limiting higher negative slpe in the higher temperature regin and vice versa. Frm the values f the limiting slpes, apparent activatin energies have been calculated. In the lwer

6 58 INDIAN J. CHEM. TECHNOL., JANUARY 2006 cm 3 /mml) frm Eq. (). The values f a are 2 and 2.5 fr Ti and Fe disslutins, respectively. S the disslutin rate cnstant, k equals t g cm 4 min - mml -2 and g cm 5.5 min - mml -2.5 fr titanium and irn disslutins, respectively. The value f a equalling t 2 fr titanium disslutin indicates that the fllwing chemical reactin is rate determining: Fig. 9 Typical [ ( R) /3 ] versus time plts fr the disslutin f bth Fe and Ti by 6 M HCl-0.5 M CH 3 OH-H 2 O mixture at different temperatures. S/L = 0.02 g/ml; stirring speed = 300 rpm. Leaching temperature: ( ), 383 K; (, ), 368 K; (, ), 348 K and (, ), 38 K. Slid symbls are fr Ti and pen fr Fe. Fig.0 lgq T versus 0/T (K - ) plts fr disslutins f Fe and Ti frm ilmenite. Leaching agent, 6 M HCl-0.5 M CH 3 OH-H 2 O mixture; time = 6 h; stirring speed = 300 rpm and S/L = 0.02 g/ml. temperature regins (~ 50 C), the activatin energies are 6.5 and 2.76 KJ ml - fr titanium and irn disslutins, respectively, whereas the respective values in the higher temperature regin are 66 and 90 KJ ml -, respectively. Density, ρ f the pwdered ilmenite sample was estimated t be 4.72 g cm -3, picnmetrically. The average radius f the particle used in leaching may be cnsidered t be cm, since it varied frm t cm. S the value f ρr 0 becmes equal t g/cm 2. The intercepts f the straight lines in Fig. 8 are 3.95 and 3.90 fr the disslutins f Ti and Fe, respectively. Since I = lg(k/ρr 0 ) frm Eq. (0), the magnitude f the disslutin rate cnstant, k cmes ut as and fr Ti and Fe, respectively. Hwever, the unit f k apparently depends n the value f a in Eq. () r (0). It is virtually g (cm 3 /mml) a min - cm -2 (since dm 3 /ml TiFeO3 + 2HCl TiOCl 2 + H 2O + FeO (3) On the ther hand, the value f a equalling t 2.5 fr irn disslutin indicates the fllwing tw chemical reactins are rate determining: FeO + 2HCl FeCl2 + H2O (4) Fe O + 3HCl FeCl + H O (5) When bth reactins represented by Eqs (4) and (5) cntribute equal t rate, then the expnent f the hydrchlric acid cncentratin term in the rate equatin will be 2.5. The abve statement is valid nly at higher temperature f leaching since the activatin energies f very high values are btained in the higher temperature regin. Hwever, as the temperature f leaching decreases (say at arund 325 K), very lw activatin energies (~6.5 KJ ml - ) are btained. The lw activatin energies in the lwer temperature regin indicate that the diffusin f HCl t the ilmenite matrix is the rate determining in lieu by the chemical reactins given abve. Cnclusins The fllwing cnclusins can be drawn frm the present investigatin: Hydrchlric acid-methanl mixture is fund t be a gd leachant than that f nitric acid-methanlwater, sulphuric acid-methanl-water r HCl alne fr the disslutin f ilmenite. The effective cncentratins f HCl and CH 3 OH fr the maximum wt % disslutin f bth metals frm ilmenite are 6 M and 0.5 M, respectively. Optimum cnditin fr leaching with 6 M HCl-0.5 M CH 3 OH-H 2 O mixture is at biling temperature and S/L rati f 0.02 g/ml. Under this cnditin 9% Ti and 95% Fe are disslved frm ilmenite in cmparisn t the disslutin f 53.5% Ti and 60% Fe by 6 M HCl alne. The high values f activatin energies in the higher temperature regin indicate the rate f the disslutin reactin is chemically cntrlled.

7 HABIB et al.: LEACHING OF NON-TREATED ILMENITE BY HCl-CH 3 OH-H 2 O MIXTURE AND ITS KINETICS 59 Hwever, the rate f the prcess is diffusin cntrlled in the lwer temperature regin. References Mirdha S & Islam M S, Reprt presented at the 0 th Annual Cnventin f Bangladesh Assciatin fr the Advancement f Science, Dhaka, March 22-27, Biswas M A B, Nucl Sci Appl, (978) 43, Islam F, Ali M & Akhtar S, Bangladesh J Sci Ind Res, 3 (978) Islam F & Kawnin Z, Bangladesh J Sci Ind Res, 3 (978) Islam F, Rahman H & Ali M, J Inrg Nucl Chem, 4 (979) Islam F, Biswas R K & Mustafa C M, Hydrmetal, 3 (985) Biswas R K & Mndal M G K, Hydrmetal, 7 (987) Hansn D A & Traut D E, J Met, (May)(987) Biswas R K, Habib M A & Dafader N C, Hydrmetal, 28 (992) 9. 0 Sinha H N, Prc Symp Extractive Metallurgy, Aus.I. M.M. Melburne (984) 63. Ogarawara T & dearauj R V V, Hydrmetal, 56 (2000) Lanyn M R, Lwin T & Merritt R, Hydrmetal, 5 (999) Olanipekum E, Hydrmetal, 53 (999). 4 Van Dyk J P, Vegter N M & Pistrius P C, Hydrmetal, 65 (2002) 3. 5 Mahmud M H H, Afifi A A I & Ibrahim I A, Hydrmetal, 73 (2004) Liang B, Li C, Zhang C & Zhang Y, Hydrmetal, 76 (2005) Sasikumar C, Ra D S, Srikanth S, Revikumar B, Mukhpadhyay N K & Mehrtra S P, Hydrmetal, 75 (2004) Lasheen T A I, Hydrmetal, 70 (2005) Amer A M, Hydrmetal, 67 (2002) Jana R K, Singh D D N & Ry S K, Hydrmetal, 38 (995) Yörükglu A & Girgin I, Hydrmetal, 6 (200) Ismail G, Hydrmetal, 24 (990) Habib M A, Biswas R K, Sarkar P K & Ahmed M, Bangladesh J Sci Ind Res (in press). 24 Vgel A I, A Text Bk f Quantitative Inrganic Analysis, 3 rd edn (Lngman, New Yrk, N. Y), 96, Vgel A I, A Text Bk f Quantitative Inrganic Analysis, 3 rd edn (Lngman, New Yrk, N. Y), 96, Habashi F, Principles f Extractive Metallurgy, Vl., Chapter 7 (Grdn and Breach Science Publishers, New Yrk, Lndn and Paris), Biswas R K, Islam M F & Habib M A, Indian J Eng Mat Sci, (994) 267.