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Leaching of calcined magnesite using ammonium chloride at constant pH
Hydrometallurgy 56 Ž2000. 109–123 www.elsevier.nlrlocaterhydromet
Leaching of calcined magnesite using ammonium chloride at constant pH Pavel Raschman Faculty of Metallurgy, Technical UniÕersity Kosice, Letna 9, 042 00 Kosice, SloÕak Republic Received 8 February 2000; received in revised form 7 March 2000; accepted 8 March 2000
Abstract The leaching of calcined magnesite using ammonium chloride has been tested in a pH-stat to ascertain the effect of process parameters viz. temperature, concentration of NH 4Cl, pH, and particle size and reactivity of the solid. The main purpose of the approach adopted was to keep the lixiviant composition constant during individual measurements. A simple mathematical model has been used to describe the dissolution of magnesium during leaching and to analyse the kinetic data. The apparent activation energy of leaching was found to depend on the particle size of the solid — the values 57.8 and 48.5 kJ moly1 were obtained for the leaching of particles of y100 q 90 and y180q 160 mm, respectively. It was concluded that the leaching process is controlled by the chemical reaction of MgO with Hq ions at the liquid–solid interface and by pore diffusion. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Leaching; Calcined magnesite; Kinetics
1. Introduction When solid magnesium oxide is added to an aqueous solution of ammonium chloride, the products magnesium chloride, ammonia and water are formed w1,2x: 2NH 4 Cl q MgO
™ MgCl q 2NH q H O. 2
3
2
E-mail address: [email protected] ŽP. Raschman.. 0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 0 7 8 - 5
Ž 1.
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The reaction of calcined magnesite with ammonium chloride according to Eq. Ž1. can play an important role in extraction of the magnesium w1,2x andror in recovery of the ammonia w1x. Good understanding of the mechanisms encountered is important for design as well as for an efficient operation of leaching reactors. However, there is limited kinetic data available for an engineering analysis of this process — it was observed that the rate of reaction Ž1. strongly depends on properties of the solid as well as on reaction conditions of the process itself and different conclusions regarding the mechanism have been presented w2–7x. Tkacova et al. w3–5x analysed the effect of physico-chemical characteristics of the solid on the rate of reaction Ž1. and yield of the magnesium. They investigated the behaviour of a large number of magnesium oxide samples prepared by various methods, and found that the feed with optimum reactivity and yield parameters can be prepared combining thermal decomposition of magnesite with screening w3x andror mechanical activation w4x. Though the authors focused on studying pre-leach treatment methods rather than the leaching process itself and analysed the effect of properties of the solid on the reaction rate only qualitatively, their results confirmed a principal difference in behaviour between porous and non-porous solids during leaching w4,5x. Glaser et al. w6,7x investigated the kinetics of reaction Ž1. using non-porous pure Žchemical. MgO w6x and porous calcined natural magnesite w7x. It was observed that: Ža. the reaction rate does not depend on whether magnesium oxide has been hydrated before use or not w7x; Žb. reaction Ž1. can be accelerated by sparging air through the reactor under certain conditions w6x; Žc. the reaction rate strongly depends on both temperature Žfrom 508C to 1008C. w6x and ammonium chloride concentration Žfrom f 0.5 to 4 M. w6,7x; Žd. pore diffusion plays an important role only for grains of calcined magnesite larger than 0.3 mm w7x. The authors proposed a reaction mechanism and derived a rate expression for the overall process Žreaction Ž1... The apparent activation energy was found to be 39 kJ moly1 and it was concluded that the physical dissolution of solid MgŽOH. 2 is the rate-controlling step of the reaction under study w6,7x. Ranjitham and Khangaonkar w2x carried out numerous measurements of the rate of the chemical reaction Ž1., studying the leaching behaviour of calcined magnesite. It was observed that the reaction rate: Ža. is sensitive to reaction temperature Žfrom 38.58C to 808C., calcination temperature Žfrom 7008C to 9008Cr3 h., particle size Žfrom 45 to 100 mm. and concentration of calcium chloride Žfrom 0 to 0.14 M.; Žb. does not depend on either prior hydration or concentration of ammonium chloride used Žfrom 2.48 to 5 M.. The variation of pH during the progress of reaction indicated a pH of 4.5 at the start, increasing to 5.5 and 6.7 at 50% and 90% completion of the reaction, respectively. The unreacted shrinking core model was applied and it was concluded that the rate of leaching is controlled by a surface chemical reaction with the activation energy 43.2 " 0.5 kJ moly1 . The aim of the present work was to contribute to better understanding of the leaching behaviour of calcined magnesite by ammonium chloride. The experimental method applied in this study made it possible to keep temperature, concentration of NH 4 Cl and pH constant during individual kinetic measurements. The process parameters investigated include temperature, concentration of NH 4 Cl, pH, and particle size and reactivity of the solid.
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2. Theoretical Dissolution of the magnesium during the leaching of calcined magnesite in ammonium chloride solution according to Eq. Ž2.: 2NH 4 Cl Ž aq. . q MgO Ž s .
™ MgCl Ž aq.. q 2NH Ž aq.. q H OŽ l. 2
3
Ž 2.
2
is a liquid–solid reaction in which no solid product is formed w2x. In the case of non-porous solids, the overall process may be controlled by intrinsic chemical reaction or by external mass transfer, while in the case of porous solids, the overall rate may be strongly influenced also by diffusion of liquid species within the pores of the solid w8x. In general, calcined natural magnesite is a porous multiphase solid w3–5x and its porosity and heterogeneity can make the use of traditional mechanistic mathematical models of liquid–solid reactions rather complicated. A simple empirical conversion–time Ž X–t . expression: X s 1 y exp Ž ykt n .
Ž 3.
has therefore been chosen for the present study, and was used to describe the process of leaching according to Eq. Ž2. in a dimensionless form: X s 1 y exp Ž ykXt n . ,
Ž 4. X
where: X — conversion of the ‘‘chemically soluble’’ magnesium Ž0 F X F 1.; k s ln2; t — dimensionless reaction Žleaching. time defined as the real reaction time t to the half reaction time t 0.5 ratio:
t s trt 0.5 ,
Ž 5.
and n is the only model parameter which has to be fitted by experiment. The half reaction time t 0.5 represents in this case a period of time which is necessary under certain reaction conditions to dissolve one half of the amount of ‘‘chemically soluble’’ magnesium originally present in the calcined magnesite. Using the half reaction time results in decreasing the total number of adjustable parameters in Eq. Ž3. and brings also a new insight into their physical meaning. Since the kinetics of leaching of oxide minerals is, in general, dependent upon the activity of hydrogen ions in the system w8x, pH has been included in the group of the process parameters investigated in the present study.
3. Experimental 3.1. Materials The bulk raw magnesite Žhydrocyclone concentrate. from Kosice, Slovakia was ˇ ground, calcined in a muffle furnace using a static air atmosphere and different retention times, and dry-screened. The leaching tests were carried out using three samples of semi-calcined magnesite, designated MKKŽ800r20.95, MKKŽ800r20.170 and MKKŽ800r30.95. The conditions of calcination and physico-chemical characteristics of
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Table 1 Physico-chemical characteristics of samples of semi-calcined magnesite Sample Ž1. Calcination temperature Ž8C. Ž2. Calcination time Žmin. Ž3. Particle size Ždiameter. Žmm. Ž4. Mean particle size Žmm. Ž5. Specific surface area Žm2 gy1 . Ž6. Chemical composition Žwt.%.: MgO CaO Fe 2 O 3 Al 2 O 3 SiO 2 L.O.I.
MKKŽ800r20. 95 800 20 y100q90 95 22.8
MKKŽ800r20. 170 800 20 y180q160 170 25.4
MKKŽ800r30. 95 800 30 y100q90 95 –
74.9 6.0 5.4 1.5 4.7 6.0
74.6 4.9 5.1 1.5 6.6 5.4
75.4 5.8 5.5 1.5 5.6 4.4
the samples are given in Table 1. Analytical reagent grade ammonium chloride and hydrochloric acid ŽLACHEMA., and distilled water were used in all experiments. 3.2. Leaching procedure and measuring kinetic data The experimental system for measuring the rate of leaching, shown in Fig. 1, consisted of a 1.1-L isothermal well-mixed glass batch reactor ŽFig. 1a. with a six-blade turbine Perspex impeller driven by a variable speed motor, four Perspex baffles installed
Fig. 1. Experimental apparatus: Ža. Leaching reactor Ž1 — impeller, 2 — heating coil, 3 — solid sample feeder, 4 — thermometer, 5 — combined glass electrode, 6 — liquor sample, 7 — baffles.; Žb. Leaching reactor with pH-stat Ž1 — thermostat, 2 — pH-stat, 3 — recorder..
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and a thermometer; and a pH-stat ŽLABORATORNI PRISTROJE. ŽFig. 1b.. The temperature was maintained to within 0.2 K by a glass-heating coil connected to a thermostat. Full mixing of the reactor content during the experiments was confirmed both visually Žsolid phase. and using potentiometric and conductometric methods Žliquid phase.. When the NH 4 Cl solution in the reactor had reached the required temperature, a small amount Žf 0.1 g. of calcined magnesite was added and the ammonia formed in reaction Ž2. was continuously titrated with standard hydrochloric acid to keep the concentrations of Hq, NH 4 Cl and NH 3 in the bulk aqueous phase practically constant during individual leaching tests. Originally, 15 mL samples of the reaction mixture were withdrawn from the reactor at appropriate time intervals, filtered and the contents of magnesium, calcium and iron in solution were estimated by atomic absorption spectroscopy. However, the results of these preliminary experiments have shown that, within the accuracy of the analytical methods used, further analyses of the product solutions may be excluded and an extent of reaction Ž2. can be directly determined from the consumption of HCl using: X Ž t. sq
V Ž t . rVM y s 1ys
,
Ž 6.
where: X Ž t . — fraction of ‘‘chemically soluble’’ magnesium reacted in time t; V Ž t . — standard hydrochloric acid consumed from t s 0 to t s t; VM — total volume of HCl consumed for the titration from t s 0 to t `, i.e. until the dissolution of MgO was completed; s — value of the Ca:Mg molar ratio in the liquor after leaching Ži.e. at X f 1.: s s 0.017, 0.014 and 0.020 for the samples MKK Ž800r20.95, MKKŽ800r20.170 and MKKŽ800r30.95, respectively. Leaching behaviour of the latter three samples of calcined magnesite was tested under reaction conditions which were as follows: temperature from 308C to 708C, concentration of NH 4 Cl from 0 to 2.5 M and pH from 3 to 6.
™
4. Results 4.1. Effect of temperature The rate of reaction Ž2. is very sensitive to leaching temperature. A typical situation is shown in Fig. 2 — the sample MKK Ž800r20. 95 was leached with 1.3 M NH 4 Cl at pH s 4.5 over the temperature range 30–708C. 4.2. Effect of ammonium chloride concentration Two periods were observed in the dependence of the rate of reaction Ž2. on ammonium chloride concentration: a constant rate period and a falling rate period. For example, from Fig. 3 it is seen that the rate of leaching of the sample MKK Ž800r20. 95 at 508C and pH s 4.5 is not affected by NH 4 Cl concentrations higher than f 1 M, while it rapidly decreases with decrease in NH 4 Cl concentration from f 1 to 0 M.
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Fig. 2. Effect of reaction temperature on fraction of MgO reacted Žleaching conditions: sample MKK Ž800r20. 95; c NH 4 Cl s1.3 M; pH s 4.5..
Fig. 3 also illustrates the difference between leaching of calcined magnesite by ammonium chloride and by hydrochloric acid. For example at retention time of 700 s, the value of conversion X f 1 was achieved in 0.5 M NH 4 Cl, while less than 10% magnesium was dissolved in 3.16 = 10y5 M HCl Ži.e. in the solution with c NH 4 Cl s 0 and pH s 4.5..
Fig. 3. Effect of ammonium chloride concentration on fraction of MgO reacted Žleaching conditions: sample MKK Ž800r20. 95; q s 508C; pH s 4.5..
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4.3. Effect of pH Measured data indicated the dependence of the rate of reaction Ž2. on the activity of Hq ions in bulk liquid to be very weak in the range studied — an example is shown in Fig. 4. 4.4. Effect of particle size The effect of different particle sizes Žy100 q 90 and y180q 160 mm. was studied using the samples MKK Ž800r20. 95 and MKK Ž800r20. 170. From Fig. 5 it is seen that the rate of the reaction Ž2. increases with decrease in particle size. 4.5. Effect of reactiÕity of calcined magnesite The effect of calcination time Ž20 and 30 min at 8008C. was studied using the samples MKK Ž800r20. 95 and MKK Ž800r30. 95. A typical result is shown in Fig. 6 — the rate of reaction Ž2. decreases with increase in calcination time. 4.6. Effect of magnesium chloride In selected experiments, 2–3 consequent runs were carried out under identical reaction conditions, using a product solution from each run as a lixiviant in the following one. It was observed that the rate of reaction Ž2. is not affected by MgCl 2
Fig. 4. Effect of pH on fraction of MgO reacted Žleaching conditions: sample MKK Ž800r20. 95; q s 508C; c NH 4 Cl s1.3 M..
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Fig. 5. Effect of particle size on fraction of MgO reacted Žleaching conditions: samples MKK Ž800r20. 95 and MKK Ž800r20. 170, pH s 4.5; for further information see the legend: mean particle sizerleaching temperaturerconcentration of NH 4 Cl..
present in the leaching solution under the conditions of the experiments in the present work.
Fig. 6. Effect of calcination time on fraction of MgO reacted Žleaching conditions: samples MKK Ž800r20. 95 and MKK Ž800r30. 95, pH s 4.5; for further information, see the legend: calcination timerleaching temperaturerconcentration of NH 4 Cl..
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5. Discussion 5.1. Experimental method The main purpose of the approach adopted was to eliminate possible effects of the changes in lixiviant composition during individual measurements on the rate of leaching. In this study, the concentrations of Hq, NH 4 Cl and NH 3 in the bulk aqueous phase were kept constant within 2–3% rel. during each run by continuous titration of the ammonia formed in reaction Ž2. with standard hydrochloric acid. It was found that the values of conversion X calculated from Eq. Ž6. are within the accuracy of applied analytical methods — especially for runs which are characterised by high reaction rates, i.e. when the period of time necessary to withdraw a sample and stop the reaction Ž10–15 s in this study. is not negligible in comparison with t 0.5 . However, the amount of calcined magnesite and the concentration of standard hydrochloric acid for individual runs must be chosen carefully to reach a convenient accuracy of the measured X–t data w9x. 5.2. Kinetic data Three series of kinetic leaching experiments were carried out, each for one of the samples of semi-calcined magnesite specified in Section 3: Series A: leaching of MKK Ž800r20. 95, Series B: leaching of MKK Ž800r20. 170, and Series D: leaching of MKK Ž800r30. 95. 5.2.1. Series A experiments A Rotatable Central Composite Design ŽRCCD. for three factors Žtemperature, ammonium chloride concentration and pH. and some additional leaching runs were carried out. First of all, the values of parameter t 0.5 were obtained for all runs by interpolation of X–t data. Then measured values of the conversion X were plotted against corresponding values of the dimensionless time t Žsee Eq. Ž5.. and the scatter diagram shown in Fig. 7 has been obtained. It can be seen from this graph that there is nearly a perfect correlation between X and t . A unique mathematical X– t expression can therefore be used in the whole observed range of reaction conditions. As explained earlier, Eq. Ž4. has been chosen for this purpose, and 95% confidence limits for the model parameter n were found to be 1.14 " 0.02 for the sample MKK Ž800r20. 95. It was found by analysing the regression equation of the RCCD experiment mentioned above, that the effect of pH on the half reaction time is negligible if compared with effects of ammonium chloride concentration c NH 4 Cl or temperature q ; one may write: 2 t 0.5 f 1193 y 31.20q y 206.4 c NH 4 Cl q 2.00q c NH 4 Cl q 0.2200q 2 q 28.1c NH . 4 Cl
Ž 7. Though there was a good correlation between the measured values of t 0.5 and the values predicted by Eq. Ž7. for 0.5 M F c NH 4 Cl F 2.5 M and 308C F q F 508C, significant
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Fig. 7. Conversion–time data for the Series A experiments.
differences Žup to 50% rel.. have been observed at lower concentrations and higher temperatures. The accuracy was significantly improved when an Arrhenius-type regression model in the form of Eq. Ž8.: b exp Ž yErRT . k f k 0 c NH 4 Cl
Ž 8.
was used. Comparison of Eqs. Ž3. – Ž5. yields the relationship between the ‘‘rate constant’’ k and t 0.5 : k sln2 tyn 0.5 ,
Ž 9.
and from Eqs. Ž8. and Ž9. one obtains the value of t 0.5 : b t 0.5 f Ž k 0rln2 . c NH exp Ž yErRT . 4 Cl
y1 rn
.
Ž 10 .
When Eq. Ž10. instead of the polynomial expression Ž7. was used, much stronger correlation between measured and predicted values of t 0.5 was achieved. This improvement is demonstrated in Fig. 8. The values of model parameters ErR, b, and k 0 obtained by a multiple regression are summarized in Table 2. The apparent activation energy of leaching was found to be 57.8 kJ moly1 for the sample MKK Ž800r20. 95. 5.2.2. Series B and D experiments When the measured values of X were plotted against the dimensionless time t , scatter diagrams very similar to that shown in Fig. 7 have been observed. The
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Fig. 8. Comparison of measured and predicted values of conversion of the magnesium for the Series A experiments Ž Xe sexperiment, Xm s model prediction: X m1 s Eqs. Ž4., Ž5. and Ž7.; X m2 s Eqs. Ž4., Ž5. and Ž10...
mathematical model given by Eqs. Ž4., Ž5. and Ž10. may therefore be used also to describe leaching of the samples MKK Ž800r20. 170 and MKK Ž800r30. 95; 95% confidence limits for the model parameter n being equal to 1.10 " 0.06 and 1.12 " 0.03, respectively. The values of other model parameters have been obtained in the same way as those for the Series A and are summarized in Table 2. From Table 2, it is seen that the apparent activation energy of leaching changes with the particle size — the values 57.8 and 48.5 kJ moly1 were obtained for the sample MKK Ž800r20. 95 and MKK Ž800r20. 170, respectively. The half reaction time t 0.5 depends on the reactivity of calcined magnesite — the higher the reactivity the lower the value of t 0.5 . In Fig. 9, the values of t 0.5 observed for the same leaching conditions are compared for the Series A, B and D. It can be concluded that the reactivity of the samples decreased in an order MKK Ž800r20. 95, MKK Ž800r30. 95, MKK Ž800r20. 170. Table 2 Parameters of model Ž10. fitted by the Series A, B and D experiments Sample of calcined magnesite
Er R ŽK.
b
ln k 0
n
r
MKK Ž800r20. 95 MKK Ž800r20. 170 MKK Ž800r30. 95
6949 5832 6491
0.406 0.356 0.295
15.84 12.14 14.36
1.14 1.10 1.12
0.996 0.999 1.000
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Fig. 9. Comparison of the reactivity of used samples of semi-calcined magnesite.
Ranjitham and Khangaonkar w2x found the optimum calcination temperature to be 7008C, higher temperatures being unfavourable due to formation of the unreactive periclase in their sample of magnesite. For calcined magnesite particles of y75 q 63 mm, calcined 180 min at 7008C and 8008C, half reaction times 3600 and 4500 s, respectively, can be estimated, using the data in Fig. 3, Ref. w2x for leaching by 2.48 M NH 4 Cl at 708C. In the present study, the value of t 0.5 of 32 s was observed under similar conditions, which were as follows: calcined magnesite particles of y100q 90 mm, calcined 20 min at 8008C, were leached with 1.3 M ammonium chloride at 708C and pH s 4.5. Such a large difference in half reaction time Žup to 2 orders of magnitude. may indicate much higher reactivity of the samples prepared using shorter calcination times. 5.3. Mechanism of leaching Observed values of the specific surface area of samples Žsee Table 1. are up to 3 orders in magnitude higher than the values calculated for non-porous particles of the same size. This fact indicates a high porosity of semi-calcined magnesite. The rate of the leaching process under study can therefore be controlled by external mass transfer, surface chemical reaction andror pore diffusion. The external mass transfer coefficient k LS can be calculated using an appropriate expression available in the literature; one has, e.g. w10x: k LS s ShDrL Sh s 1 q 0.38 Re
Ž 11 . 1r2
Sc
1r3
Ž 12 .
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Fig. 10. Determination of the rate-controlling step — Series A experiments.
where: D is the diffusivity of NH 4 Cl Žm2 sy1 ., L is the radius of the solid particle Žm., and Sh, Re and Sc are the Sherwood, Reynolds and Schmidt numbers, respectively. Conservative estimates of k LS being of order 1.10y4 m sy1 indicate that the resistance for external mass transfer is negligible Žup to 2 orders lower. if compared with the overall resistance corresponding to the observed overall rate — pore diffusion and chemical reaction are therefore potential rate-controlling steps. Presence of diffusional limitations was tested by a method based on the random pore X .rŽ1 y X .x 2 is plotted against ylnŽ1 y X ., a straight line model w11x. When wŽ X XrX 0.5 should be obtained if the intrinsic chemical reaction is the only rate-controlling step. The actual situation for the Series A experiments is shown in Fig. 10 and it may therefore be concluded that the overall rate of leaching is controlled by both chemical reaction of MgO with Hq ions and pore diffusion. An analogous situation has been observed for the samples MKK Ž800r30. 95 or MKK Ž800r20. 170. 6. Conclusions Kinetics of the liquid–solid reaction between calcined magnesite and ammonium chloride solution Žaccording to Eq. Ž2.. were investigated under constant pH. The main purpose of the approach adopted was to eliminate possible effects of the changes in lixiviant composition during individual runs on the rate of leaching. It was observed that the reaction rate: Ža. is very sensitive to temperature in the range 30–708C; Žb. increases rapidly with increase in NH 4 Cl concentration from 0 to f 1 M, while it is practically not affected by higher concentrations of NH 4 Cl Žup to 2.5 M.; Žc.
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is not significantly affected by pH in the range from 3 to 6; Žd. increases with decrease in particle size of calcined magnesite; Že. decreases with increase in calcination time. The results are presented in terms of an empirical mathematical model. The kinetic data indicated that the overall rate of leaching is controlled by pore diffusion and surface chemical reaction under the conditions of the experiments in the present work. The apparent activation energy of leaching was found to depend on the particle size of calcined magnesite — the values 57.8 and 48.5 kJ moly1 were obtained for the leaching of particles of y100 q 90 and y 180 q 160 mm, respectively. Presence of diffusional limitations was confirmed by a method based on the random pore model. 7. Notation b c NH 4 Cl D E k kX k0 k LS L n r R Re Sc Sh t T V X XX
parameter in Eq. Ž8. concentration of ammonium chloride in the reaction mixture Žmol dmy3 . diffusivity of NH 4 Cl Žm2 sy1 . apparent activation energy ŽJ moly1 . ‘‘rate constant’’ in Eq. Ž3. Žsyn . constant in Eq. Ž4. parameter in Eq. Ž10. Žsyn molyb dm3 b . external mass transfer coefficient Žm sy1 . radius of the solid particle Žm. parameter in Eqs. Ž3. and Ž4. coefficient of correlation gas constants 8.314 J moly1 Ky1 Reynolds number Schmidt number Sherwood number time of leaching Žs. temperature ŽK. volume of standard hydrochloric acid consumed for titration of NH 3 Žm3 . conversion Žfraction reacted. of the ‘‘chemically soluble’’ magnesium s d Xrdt
Greek symbols q temperature Ž8C. s Ca:Mg molar ratio in the liquor when leaching is completed t dimensionless time of leaching defined by Eq. Ž5. Subscripts 0.5 the value for X s 0.5 M maximum value Acknowledgements This work was supported by the Slovak Grant Agency for Science ŽGrant 2r4179r97 and 1r4361r97..
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References w1x K. Tkacova, F. Markalous, J. Sturc, L. Turcaniova, An integrated chemical plant for magnesite and salt processing, Rudy 29 Ž1981. 78–80, Žin Slovak.. w2x A.M. Ranjitham, P.R. Khangaonkar, Leaching behaviour of calcined magnesite with ammonium chloride solutions, Hydrometallurgy 23 Ž1990. 177–189. w3x B. Plesingerova, K. Tkacova, L. Turcaniova, Removal of the calcium from the raw-materials for producing pure MgO by the chloride-carbonate method, Uhlı-Rudy 9 Ž1. Ž1992. 326–329, Žin Slovak.. ´ w4x K. Tkacova, L. Turcaniova, I. Hocmanova, Effect of conditions of calcination and comminution of MgO on the hydrolytic decomposition of NH 4 Cl, Rudy 29 Ž1981. 196–202, Žin Slovak.. w5x K. Tkacova, L. Turcaniova, I. Hocmanova, Thermal and mechanical activation as alternative or combined methods of pre-leach treatment of magnesite, in: Proc. of the 2nd World Congress on Non-metallic Minerals, Beijing, 1982, pp. 876–879. w6x V. Glaser, J. Vidensky, M. Kuzela, Kinetics of the reaction between magnesium oxide and ammonium chloride solution, Collect. Czech. Chem. Commun. 53 Ž1988. 1711–1717. w7x V. Glaser, J. Vidensky, Kinetics of ammonia recovery from soda-plant liquor by burnt magnesite, Collect. Czech. Chem. Commun. 53 Ž1988. 1718–1724. w8x H.L.Y. Sohn, M.E. Wadsworth ŽEds.., Rate Processes of Extractive Metallurgy Plenum, New York, 1979. w9x P. Raschman, Laboratory method for measuring the reactivity of calcined magnesite, in: Proc. of the Int. Conf. New trends in Mineral Processing II, VSB-Technical University Ostrava, 1997, pp. 252–258, Žin Slovak.. w10x C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, Khimiya, Moscow, 1976, ŽRussian edition.. w11x M. Sahimi, G.R. Gavalas, T. Tsotsis, Statistical and continuum models of fluid–solid reactions in porous media, Chem. Eng. Sci. 45 Ž1990. 1449–1452.
Leaching of calcined magnesite using ammonium chloride at constant pH Pavel Raschman Faculty of Metallurgy, Technical UniÕersity Kosice, Letna 9, 042 00 Kosice, SloÕak Republic Received 8 February 2000; received in revised form 7 March 2000; accepted 8 March 2000
Abstract The leaching of calcined magnesite using ammonium chloride has been tested in a pH-stat to ascertain the effect of process parameters viz. temperature, concentration of NH 4Cl, pH, and particle size and reactivity of the solid. The main purpose of the approach adopted was to keep the lixiviant composition constant during individual measurements. A simple mathematical model has been used to describe the dissolution of magnesium during leaching and to analyse the kinetic data. The apparent activation energy of leaching was found to depend on the particle size of the solid — the values 57.8 and 48.5 kJ moly1 were obtained for the leaching of particles of y100 q 90 and y180q 160 mm, respectively. It was concluded that the leaching process is controlled by the chemical reaction of MgO with Hq ions at the liquid–solid interface and by pore diffusion. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Leaching; Calcined magnesite; Kinetics
1. Introduction When solid magnesium oxide is added to an aqueous solution of ammonium chloride, the products magnesium chloride, ammonia and water are formed w1,2x: 2NH 4 Cl q MgO
™ MgCl q 2NH q H O. 2
3
2
E-mail address: [email protected] ŽP. Raschman.. 0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 0 . 0 0 0 7 8 - 5
Ž 1.
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The reaction of calcined magnesite with ammonium chloride according to Eq. Ž1. can play an important role in extraction of the magnesium w1,2x andror in recovery of the ammonia w1x. Good understanding of the mechanisms encountered is important for design as well as for an efficient operation of leaching reactors. However, there is limited kinetic data available for an engineering analysis of this process — it was observed that the rate of reaction Ž1. strongly depends on properties of the solid as well as on reaction conditions of the process itself and different conclusions regarding the mechanism have been presented w2–7x. Tkacova et al. w3–5x analysed the effect of physico-chemical characteristics of the solid on the rate of reaction Ž1. and yield of the magnesium. They investigated the behaviour of a large number of magnesium oxide samples prepared by various methods, and found that the feed with optimum reactivity and yield parameters can be prepared combining thermal decomposition of magnesite with screening w3x andror mechanical activation w4x. Though the authors focused on studying pre-leach treatment methods rather than the leaching process itself and analysed the effect of properties of the solid on the reaction rate only qualitatively, their results confirmed a principal difference in behaviour between porous and non-porous solids during leaching w4,5x. Glaser et al. w6,7x investigated the kinetics of reaction Ž1. using non-porous pure Žchemical. MgO w6x and porous calcined natural magnesite w7x. It was observed that: Ža. the reaction rate does not depend on whether magnesium oxide has been hydrated before use or not w7x; Žb. reaction Ž1. can be accelerated by sparging air through the reactor under certain conditions w6x; Žc. the reaction rate strongly depends on both temperature Žfrom 508C to 1008C. w6x and ammonium chloride concentration Žfrom f 0.5 to 4 M. w6,7x; Žd. pore diffusion plays an important role only for grains of calcined magnesite larger than 0.3 mm w7x. The authors proposed a reaction mechanism and derived a rate expression for the overall process Žreaction Ž1... The apparent activation energy was found to be 39 kJ moly1 and it was concluded that the physical dissolution of solid MgŽOH. 2 is the rate-controlling step of the reaction under study w6,7x. Ranjitham and Khangaonkar w2x carried out numerous measurements of the rate of the chemical reaction Ž1., studying the leaching behaviour of calcined magnesite. It was observed that the reaction rate: Ža. is sensitive to reaction temperature Žfrom 38.58C to 808C., calcination temperature Žfrom 7008C to 9008Cr3 h., particle size Žfrom 45 to 100 mm. and concentration of calcium chloride Žfrom 0 to 0.14 M.; Žb. does not depend on either prior hydration or concentration of ammonium chloride used Žfrom 2.48 to 5 M.. The variation of pH during the progress of reaction indicated a pH of 4.5 at the start, increasing to 5.5 and 6.7 at 50% and 90% completion of the reaction, respectively. The unreacted shrinking core model was applied and it was concluded that the rate of leaching is controlled by a surface chemical reaction with the activation energy 43.2 " 0.5 kJ moly1 . The aim of the present work was to contribute to better understanding of the leaching behaviour of calcined magnesite by ammonium chloride. The experimental method applied in this study made it possible to keep temperature, concentration of NH 4 Cl and pH constant during individual kinetic measurements. The process parameters investigated include temperature, concentration of NH 4 Cl, pH, and particle size and reactivity of the solid.
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2. Theoretical Dissolution of the magnesium during the leaching of calcined magnesite in ammonium chloride solution according to Eq. Ž2.: 2NH 4 Cl Ž aq. . q MgO Ž s .
™ MgCl Ž aq.. q 2NH Ž aq.. q H OŽ l. 2
3
Ž 2.
2
is a liquid–solid reaction in which no solid product is formed w2x. In the case of non-porous solids, the overall process may be controlled by intrinsic chemical reaction or by external mass transfer, while in the case of porous solids, the overall rate may be strongly influenced also by diffusion of liquid species within the pores of the solid w8x. In general, calcined natural magnesite is a porous multiphase solid w3–5x and its porosity and heterogeneity can make the use of traditional mechanistic mathematical models of liquid–solid reactions rather complicated. A simple empirical conversion–time Ž X–t . expression: X s 1 y exp Ž ykt n .
Ž 3.
has therefore been chosen for the present study, and was used to describe the process of leaching according to Eq. Ž2. in a dimensionless form: X s 1 y exp Ž ykXt n . ,
Ž 4. X
where: X — conversion of the ‘‘chemically soluble’’ magnesium Ž0 F X F 1.; k s ln2; t — dimensionless reaction Žleaching. time defined as the real reaction time t to the half reaction time t 0.5 ratio:
t s trt 0.5 ,
Ž 5.
and n is the only model parameter which has to be fitted by experiment. The half reaction time t 0.5 represents in this case a period of time which is necessary under certain reaction conditions to dissolve one half of the amount of ‘‘chemically soluble’’ magnesium originally present in the calcined magnesite. Using the half reaction time results in decreasing the total number of adjustable parameters in Eq. Ž3. and brings also a new insight into their physical meaning. Since the kinetics of leaching of oxide minerals is, in general, dependent upon the activity of hydrogen ions in the system w8x, pH has been included in the group of the process parameters investigated in the present study.
3. Experimental 3.1. Materials The bulk raw magnesite Žhydrocyclone concentrate. from Kosice, Slovakia was ˇ ground, calcined in a muffle furnace using a static air atmosphere and different retention times, and dry-screened. The leaching tests were carried out using three samples of semi-calcined magnesite, designated MKKŽ800r20.95, MKKŽ800r20.170 and MKKŽ800r30.95. The conditions of calcination and physico-chemical characteristics of
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Table 1 Physico-chemical characteristics of samples of semi-calcined magnesite Sample Ž1. Calcination temperature Ž8C. Ž2. Calcination time Žmin. Ž3. Particle size Ždiameter. Žmm. Ž4. Mean particle size Žmm. Ž5. Specific surface area Žm2 gy1 . Ž6. Chemical composition Žwt.%.: MgO CaO Fe 2 O 3 Al 2 O 3 SiO 2 L.O.I.
MKKŽ800r20. 95 800 20 y100q90 95 22.8
MKKŽ800r20. 170 800 20 y180q160 170 25.4
MKKŽ800r30. 95 800 30 y100q90 95 –
74.9 6.0 5.4 1.5 4.7 6.0
74.6 4.9 5.1 1.5 6.6 5.4
75.4 5.8 5.5 1.5 5.6 4.4
the samples are given in Table 1. Analytical reagent grade ammonium chloride and hydrochloric acid ŽLACHEMA., and distilled water were used in all experiments. 3.2. Leaching procedure and measuring kinetic data The experimental system for measuring the rate of leaching, shown in Fig. 1, consisted of a 1.1-L isothermal well-mixed glass batch reactor ŽFig. 1a. with a six-blade turbine Perspex impeller driven by a variable speed motor, four Perspex baffles installed
Fig. 1. Experimental apparatus: Ža. Leaching reactor Ž1 — impeller, 2 — heating coil, 3 — solid sample feeder, 4 — thermometer, 5 — combined glass electrode, 6 — liquor sample, 7 — baffles.; Žb. Leaching reactor with pH-stat Ž1 — thermostat, 2 — pH-stat, 3 — recorder..
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and a thermometer; and a pH-stat ŽLABORATORNI PRISTROJE. ŽFig. 1b.. The temperature was maintained to within 0.2 K by a glass-heating coil connected to a thermostat. Full mixing of the reactor content during the experiments was confirmed both visually Žsolid phase. and using potentiometric and conductometric methods Žliquid phase.. When the NH 4 Cl solution in the reactor had reached the required temperature, a small amount Žf 0.1 g. of calcined magnesite was added and the ammonia formed in reaction Ž2. was continuously titrated with standard hydrochloric acid to keep the concentrations of Hq, NH 4 Cl and NH 3 in the bulk aqueous phase practically constant during individual leaching tests. Originally, 15 mL samples of the reaction mixture were withdrawn from the reactor at appropriate time intervals, filtered and the contents of magnesium, calcium and iron in solution were estimated by atomic absorption spectroscopy. However, the results of these preliminary experiments have shown that, within the accuracy of the analytical methods used, further analyses of the product solutions may be excluded and an extent of reaction Ž2. can be directly determined from the consumption of HCl using: X Ž t. sq
V Ž t . rVM y s 1ys
,
Ž 6.
where: X Ž t . — fraction of ‘‘chemically soluble’’ magnesium reacted in time t; V Ž t . — standard hydrochloric acid consumed from t s 0 to t s t; VM — total volume of HCl consumed for the titration from t s 0 to t `, i.e. until the dissolution of MgO was completed; s — value of the Ca:Mg molar ratio in the liquor after leaching Ži.e. at X f 1.: s s 0.017, 0.014 and 0.020 for the samples MKK Ž800r20.95, MKKŽ800r20.170 and MKKŽ800r30.95, respectively. Leaching behaviour of the latter three samples of calcined magnesite was tested under reaction conditions which were as follows: temperature from 308C to 708C, concentration of NH 4 Cl from 0 to 2.5 M and pH from 3 to 6.
™
4. Results 4.1. Effect of temperature The rate of reaction Ž2. is very sensitive to leaching temperature. A typical situation is shown in Fig. 2 — the sample MKK Ž800r20. 95 was leached with 1.3 M NH 4 Cl at pH s 4.5 over the temperature range 30–708C. 4.2. Effect of ammonium chloride concentration Two periods were observed in the dependence of the rate of reaction Ž2. on ammonium chloride concentration: a constant rate period and a falling rate period. For example, from Fig. 3 it is seen that the rate of leaching of the sample MKK Ž800r20. 95 at 508C and pH s 4.5 is not affected by NH 4 Cl concentrations higher than f 1 M, while it rapidly decreases with decrease in NH 4 Cl concentration from f 1 to 0 M.
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Fig. 2. Effect of reaction temperature on fraction of MgO reacted Žleaching conditions: sample MKK Ž800r20. 95; c NH 4 Cl s1.3 M; pH s 4.5..
Fig. 3 also illustrates the difference between leaching of calcined magnesite by ammonium chloride and by hydrochloric acid. For example at retention time of 700 s, the value of conversion X f 1 was achieved in 0.5 M NH 4 Cl, while less than 10% magnesium was dissolved in 3.16 = 10y5 M HCl Ži.e. in the solution with c NH 4 Cl s 0 and pH s 4.5..
Fig. 3. Effect of ammonium chloride concentration on fraction of MgO reacted Žleaching conditions: sample MKK Ž800r20. 95; q s 508C; pH s 4.5..
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4.3. Effect of pH Measured data indicated the dependence of the rate of reaction Ž2. on the activity of Hq ions in bulk liquid to be very weak in the range studied — an example is shown in Fig. 4. 4.4. Effect of particle size The effect of different particle sizes Žy100 q 90 and y180q 160 mm. was studied using the samples MKK Ž800r20. 95 and MKK Ž800r20. 170. From Fig. 5 it is seen that the rate of the reaction Ž2. increases with decrease in particle size. 4.5. Effect of reactiÕity of calcined magnesite The effect of calcination time Ž20 and 30 min at 8008C. was studied using the samples MKK Ž800r20. 95 and MKK Ž800r30. 95. A typical result is shown in Fig. 6 — the rate of reaction Ž2. decreases with increase in calcination time. 4.6. Effect of magnesium chloride In selected experiments, 2–3 consequent runs were carried out under identical reaction conditions, using a product solution from each run as a lixiviant in the following one. It was observed that the rate of reaction Ž2. is not affected by MgCl 2
Fig. 4. Effect of pH on fraction of MgO reacted Žleaching conditions: sample MKK Ž800r20. 95; q s 508C; c NH 4 Cl s1.3 M..
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Fig. 5. Effect of particle size on fraction of MgO reacted Žleaching conditions: samples MKK Ž800r20. 95 and MKK Ž800r20. 170, pH s 4.5; for further information see the legend: mean particle sizerleaching temperaturerconcentration of NH 4 Cl..
present in the leaching solution under the conditions of the experiments in the present work.
Fig. 6. Effect of calcination time on fraction of MgO reacted Žleaching conditions: samples MKK Ž800r20. 95 and MKK Ž800r30. 95, pH s 4.5; for further information, see the legend: calcination timerleaching temperaturerconcentration of NH 4 Cl..
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5. Discussion 5.1. Experimental method The main purpose of the approach adopted was to eliminate possible effects of the changes in lixiviant composition during individual measurements on the rate of leaching. In this study, the concentrations of Hq, NH 4 Cl and NH 3 in the bulk aqueous phase were kept constant within 2–3% rel. during each run by continuous titration of the ammonia formed in reaction Ž2. with standard hydrochloric acid. It was found that the values of conversion X calculated from Eq. Ž6. are within the accuracy of applied analytical methods — especially for runs which are characterised by high reaction rates, i.e. when the period of time necessary to withdraw a sample and stop the reaction Ž10–15 s in this study. is not negligible in comparison with t 0.5 . However, the amount of calcined magnesite and the concentration of standard hydrochloric acid for individual runs must be chosen carefully to reach a convenient accuracy of the measured X–t data w9x. 5.2. Kinetic data Three series of kinetic leaching experiments were carried out, each for one of the samples of semi-calcined magnesite specified in Section 3: Series A: leaching of MKK Ž800r20. 95, Series B: leaching of MKK Ž800r20. 170, and Series D: leaching of MKK Ž800r30. 95. 5.2.1. Series A experiments A Rotatable Central Composite Design ŽRCCD. for three factors Žtemperature, ammonium chloride concentration and pH. and some additional leaching runs were carried out. First of all, the values of parameter t 0.5 were obtained for all runs by interpolation of X–t data. Then measured values of the conversion X were plotted against corresponding values of the dimensionless time t Žsee Eq. Ž5.. and the scatter diagram shown in Fig. 7 has been obtained. It can be seen from this graph that there is nearly a perfect correlation between X and t . A unique mathematical X– t expression can therefore be used in the whole observed range of reaction conditions. As explained earlier, Eq. Ž4. has been chosen for this purpose, and 95% confidence limits for the model parameter n were found to be 1.14 " 0.02 for the sample MKK Ž800r20. 95. It was found by analysing the regression equation of the RCCD experiment mentioned above, that the effect of pH on the half reaction time is negligible if compared with effects of ammonium chloride concentration c NH 4 Cl or temperature q ; one may write: 2 t 0.5 f 1193 y 31.20q y 206.4 c NH 4 Cl q 2.00q c NH 4 Cl q 0.2200q 2 q 28.1c NH . 4 Cl
Ž 7. Though there was a good correlation between the measured values of t 0.5 and the values predicted by Eq. Ž7. for 0.5 M F c NH 4 Cl F 2.5 M and 308C F q F 508C, significant
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Fig. 7. Conversion–time data for the Series A experiments.
differences Žup to 50% rel.. have been observed at lower concentrations and higher temperatures. The accuracy was significantly improved when an Arrhenius-type regression model in the form of Eq. Ž8.: b exp Ž yErRT . k f k 0 c NH 4 Cl
Ž 8.
was used. Comparison of Eqs. Ž3. – Ž5. yields the relationship between the ‘‘rate constant’’ k and t 0.5 : k sln2 tyn 0.5 ,
Ž 9.
and from Eqs. Ž8. and Ž9. one obtains the value of t 0.5 : b t 0.5 f Ž k 0rln2 . c NH exp Ž yErRT . 4 Cl
y1 rn
.
Ž 10 .
When Eq. Ž10. instead of the polynomial expression Ž7. was used, much stronger correlation between measured and predicted values of t 0.5 was achieved. This improvement is demonstrated in Fig. 8. The values of model parameters ErR, b, and k 0 obtained by a multiple regression are summarized in Table 2. The apparent activation energy of leaching was found to be 57.8 kJ moly1 for the sample MKK Ž800r20. 95. 5.2.2. Series B and D experiments When the measured values of X were plotted against the dimensionless time t , scatter diagrams very similar to that shown in Fig. 7 have been observed. The
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Fig. 8. Comparison of measured and predicted values of conversion of the magnesium for the Series A experiments Ž Xe sexperiment, Xm s model prediction: X m1 s Eqs. Ž4., Ž5. and Ž7.; X m2 s Eqs. Ž4., Ž5. and Ž10...
mathematical model given by Eqs. Ž4., Ž5. and Ž10. may therefore be used also to describe leaching of the samples MKK Ž800r20. 170 and MKK Ž800r30. 95; 95% confidence limits for the model parameter n being equal to 1.10 " 0.06 and 1.12 " 0.03, respectively. The values of other model parameters have been obtained in the same way as those for the Series A and are summarized in Table 2. From Table 2, it is seen that the apparent activation energy of leaching changes with the particle size — the values 57.8 and 48.5 kJ moly1 were obtained for the sample MKK Ž800r20. 95 and MKK Ž800r20. 170, respectively. The half reaction time t 0.5 depends on the reactivity of calcined magnesite — the higher the reactivity the lower the value of t 0.5 . In Fig. 9, the values of t 0.5 observed for the same leaching conditions are compared for the Series A, B and D. It can be concluded that the reactivity of the samples decreased in an order MKK Ž800r20. 95, MKK Ž800r30. 95, MKK Ž800r20. 170. Table 2 Parameters of model Ž10. fitted by the Series A, B and D experiments Sample of calcined magnesite
Er R ŽK.
b
ln k 0
n
r
MKK Ž800r20. 95 MKK Ž800r20. 170 MKK Ž800r30. 95
6949 5832 6491
0.406 0.356 0.295
15.84 12.14 14.36
1.14 1.10 1.12
0.996 0.999 1.000
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Fig. 9. Comparison of the reactivity of used samples of semi-calcined magnesite.
Ranjitham and Khangaonkar w2x found the optimum calcination temperature to be 7008C, higher temperatures being unfavourable due to formation of the unreactive periclase in their sample of magnesite. For calcined magnesite particles of y75 q 63 mm, calcined 180 min at 7008C and 8008C, half reaction times 3600 and 4500 s, respectively, can be estimated, using the data in Fig. 3, Ref. w2x for leaching by 2.48 M NH 4 Cl at 708C. In the present study, the value of t 0.5 of 32 s was observed under similar conditions, which were as follows: calcined magnesite particles of y100q 90 mm, calcined 20 min at 8008C, were leached with 1.3 M ammonium chloride at 708C and pH s 4.5. Such a large difference in half reaction time Žup to 2 orders of magnitude. may indicate much higher reactivity of the samples prepared using shorter calcination times. 5.3. Mechanism of leaching Observed values of the specific surface area of samples Žsee Table 1. are up to 3 orders in magnitude higher than the values calculated for non-porous particles of the same size. This fact indicates a high porosity of semi-calcined magnesite. The rate of the leaching process under study can therefore be controlled by external mass transfer, surface chemical reaction andror pore diffusion. The external mass transfer coefficient k LS can be calculated using an appropriate expression available in the literature; one has, e.g. w10x: k LS s ShDrL Sh s 1 q 0.38 Re
Ž 11 . 1r2
Sc
1r3
Ž 12 .
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Fig. 10. Determination of the rate-controlling step — Series A experiments.
where: D is the diffusivity of NH 4 Cl Žm2 sy1 ., L is the radius of the solid particle Žm., and Sh, Re and Sc are the Sherwood, Reynolds and Schmidt numbers, respectively. Conservative estimates of k LS being of order 1.10y4 m sy1 indicate that the resistance for external mass transfer is negligible Žup to 2 orders lower. if compared with the overall resistance corresponding to the observed overall rate — pore diffusion and chemical reaction are therefore potential rate-controlling steps. Presence of diffusional limitations was tested by a method based on the random pore X .rŽ1 y X .x 2 is plotted against ylnŽ1 y X ., a straight line model w11x. When wŽ X XrX 0.5 should be obtained if the intrinsic chemical reaction is the only rate-controlling step. The actual situation for the Series A experiments is shown in Fig. 10 and it may therefore be concluded that the overall rate of leaching is controlled by both chemical reaction of MgO with Hq ions and pore diffusion. An analogous situation has been observed for the samples MKK Ž800r30. 95 or MKK Ž800r20. 170. 6. Conclusions Kinetics of the liquid–solid reaction between calcined magnesite and ammonium chloride solution Žaccording to Eq. Ž2.. were investigated under constant pH. The main purpose of the approach adopted was to eliminate possible effects of the changes in lixiviant composition during individual runs on the rate of leaching. It was observed that the reaction rate: Ža. is very sensitive to temperature in the range 30–708C; Žb. increases rapidly with increase in NH 4 Cl concentration from 0 to f 1 M, while it is practically not affected by higher concentrations of NH 4 Cl Žup to 2.5 M.; Žc.
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is not significantly affected by pH in the range from 3 to 6; Žd. increases with decrease in particle size of calcined magnesite; Že. decreases with increase in calcination time. The results are presented in terms of an empirical mathematical model. The kinetic data indicated that the overall rate of leaching is controlled by pore diffusion and surface chemical reaction under the conditions of the experiments in the present work. The apparent activation energy of leaching was found to depend on the particle size of calcined magnesite — the values 57.8 and 48.5 kJ moly1 were obtained for the leaching of particles of y100 q 90 and y 180 q 160 mm, respectively. Presence of diffusional limitations was confirmed by a method based on the random pore model. 7. Notation b c NH 4 Cl D E k kX k0 k LS L n r R Re Sc Sh t T V X XX
parameter in Eq. Ž8. concentration of ammonium chloride in the reaction mixture Žmol dmy3 . diffusivity of NH 4 Cl Žm2 sy1 . apparent activation energy ŽJ moly1 . ‘‘rate constant’’ in Eq. Ž3. Žsyn . constant in Eq. Ž4. parameter in Eq. Ž10. Žsyn molyb dm3 b . external mass transfer coefficient Žm sy1 . radius of the solid particle Žm. parameter in Eqs. Ž3. and Ž4. coefficient of correlation gas constants 8.314 J moly1 Ky1 Reynolds number Schmidt number Sherwood number time of leaching Žs. temperature ŽK. volume of standard hydrochloric acid consumed for titration of NH 3 Žm3 . conversion Žfraction reacted. of the ‘‘chemically soluble’’ magnesium s d Xrdt
Greek symbols q temperature Ž8C. s Ca:Mg molar ratio in the liquor when leaching is completed t dimensionless time of leaching defined by Eq. Ž5. Subscripts 0.5 the value for X s 0.5 M maximum value Acknowledgements This work was supported by the Slovak Grant Agency for Science ŽGrant 2r4179r97 and 1r4361r97..
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References w1x K. Tkacova, F. Markalous, J. Sturc, L. Turcaniova, An integrated chemical plant for magnesite and salt processing, Rudy 29 Ž1981. 78–80, Žin Slovak.. w2x A.M. Ranjitham, P.R. Khangaonkar, Leaching behaviour of calcined magnesite with ammonium chloride solutions, Hydrometallurgy 23 Ž1990. 177–189. w3x B. Plesingerova, K. Tkacova, L. Turcaniova, Removal of the calcium from the raw-materials for producing pure MgO by the chloride-carbonate method, Uhlı-Rudy 9 Ž1. Ž1992. 326–329, Žin Slovak.. ´ w4x K. Tkacova, L. Turcaniova, I. Hocmanova, Effect of conditions of calcination and comminution of MgO on the hydrolytic decomposition of NH 4 Cl, Rudy 29 Ž1981. 196–202, Žin Slovak.. w5x K. Tkacova, L. Turcaniova, I. Hocmanova, Thermal and mechanical activation as alternative or combined methods of pre-leach treatment of magnesite, in: Proc. of the 2nd World Congress on Non-metallic Minerals, Beijing, 1982, pp. 876–879. w6x V. Glaser, J. Vidensky, M. Kuzela, Kinetics of the reaction between magnesium oxide and ammonium chloride solution, Collect. Czech. Chem. Commun. 53 Ž1988. 1711–1717. w7x V. Glaser, J. Vidensky, Kinetics of ammonia recovery from soda-plant liquor by burnt magnesite, Collect. Czech. Chem. Commun. 53 Ž1988. 1718–1724. w8x H.L.Y. Sohn, M.E. Wadsworth ŽEds.., Rate Processes of Extractive Metallurgy Plenum, New York, 1979. w9x P. Raschman, Laboratory method for measuring the reactivity of calcined magnesite, in: Proc. of the Int. Conf. New trends in Mineral Processing II, VSB-Technical University Ostrava, 1997, pp. 252–258, Žin Slovak.. w10x C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, Khimiya, Moscow, 1976, ŽRussian edition.. w11x M. Sahimi, G.R. Gavalas, T. Tsotsis, Statistical and continuum models of fluid–solid reactions in porous media, Chem. Eng. Sci. 45 Ž1990. 1449–1452.