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Studies on the extraction of magnesia from low grade magnesites by carbon dioxide pressure leaching of hydrated magnesia
Minerals Engineering, Vol. 4, No. 1, pp. 7 9 - 8 8 , 1991
0892-6875/91 $3.00 + IX) © 1990 Pergamon Press plc
Printed in Great Britain
STUDIES ON THE EXTRACTION OF MAGNESIA FROM LOW GRADE MAGNESITES BY CARBON DIOXIDE PRESSURE LEACHING OF HYDRATED MAGNESIA D. SHEILA§, C. SANKARANI and P.R. KHANGAONKARI
§ Centre for Water Resources, Guindy, Engineering College, Madras 60025, India t National Metallurgical Laboratory, Madras Centre, CSIR Madras Complex, India School of Material and Mineral Resources Engng., University Sains Malaysia, Perak Branch Campus, 30000 Ipoh, Malaysia (Received 6 February 1990; revision accepted 4 May 1990)
ABSTRACT
The applicability of carbon dioxide pressure leaching for the extraction of magnesia of high purity from low grade magnesites and waste dumps material has been investigated. Factors which influence the magnesia recovery such as time, temperature, pressure, particle size, solid~liquid ratio and calcination temperature have been studied and optimum values were determined. The kinetic data were examined according to therate equation -In(L- ~) against t and the activation energy of 26.4 kJ tool -1 (6.3Kcal tool "1) suggested that the kinetics o f the reaction are controlled by surface chemical reaction. Keywords Magnesite; carbon dioxide; hydration; pressure leaching; solubility; kinetics INTRODUCTION
Magnesite occurs extensively in Tamilnadu, Uttar Pradesh, Rajasthan, Kashmir and Karnataka in India. These natural magnesite deposits are contaminated with impurities like silica, iron oxides, alumina etc., and need to be suitably beneficiated prior to its use in refractories and in the preparation of magnesium compounds. Beneficiation of magnesite under Indian practice mostly consists of selective mining, screening and hand sorting to produce a marketable grade of magnesite with less than 4% SiO 2. Such a practice results in poor magnesite recoveries (30-35%) and substantial quantities are dumped. This has resulted in the piling up of huge magnesite dumps near the mine site. The rapid growth in the steel industry and particularly the increased use of tonnage oxygen in steel making has created heavy demands for high grade magnesite refractories. In order to cope with the increasing demand for high purity sintered magnesia, increasing attention is being paid throughout the world to the upgrading of low-grade and submarginal magnesites either by beneficiation or chemical treatment. The current supplies of high quality magnesia are derived from the processing of selected high grades of magnesite, low in SIO2, or from sea water. The basic oxygen converter and electric arc furnace steel making processes cannot accept low grade sintered magnesia as refractories and thus they depend heavily on sea water magnesia for their supplies. Beneficiation of magnesite would be the obvious solution to the need for alternative sources. 79
80
D. SHEILAet al.
Physical methods of beneficiation of magnesite [1-3] rely on the physical differences between magnesium carbonate and the impurities to effect the separation i.e. density, optical properties, gravity, flotation etc. Though these methods are economic, unfortunately they fail to achieve the desired results due to various limitations of the physical factors involved. Chemical beneficiation, relies on chemical interaction and selectivity of leaching of magnesia that results in its removal in a state of high purity. Many chemical methods are available to obtain magnesia of high purity from magnesite. The choice of a leachant depends on its selectivity and the nature of impurities. The use of common acids, like hydrochloric, sulphuric or nitric acids [4-6] has been examined, but has not been commercialised because of the complex purification and acid regeneration/recovery steps. A m m o n i u m chloride or calcium chloride leaching in the presence of carbon dioxide has been successfully used to selectively leach magnesia from calcined magnesite. The resulting magnesium chloride is treated with ammonia and carbon dioxide to precipitate magnesium carbonate which is then calcined, briquetted and sintered to produce high purity sintered magnesia. The calcium chloride leaching of the calcined magnesia in the presence of carbon dioxide forms the basis of the recently developed Sulmag (R) [7] process and all the reagents used at various stages in the process are recovered and suitably recycled. Carbon dioxide [8-21] has been found to selectively dissolve magnesia forming a soluble magnesium bicarbonate, leaving the impurities unaffected. The solution is decomposed by heating and/or aeration to form basic magnesium carbonate which is then suitably processed to obtain high purity magnesia. The major disadvantage of the carbon dioxide process is the relatively low solubility of magnesium bicarbonate resulting in increased capital costs when processing huge volumes of solutions for a given throughput of magnesia. High carbon dioxide pressures and low temperatures have been reported to increase the solubility of magnesium bicarbonate [22]. Despite the simplicity of the aqueous pressure carbonation process for the recovery of high purity magnesia, not much work has been done on Indian magnesites and waste dumps at the ambient temperatures (~303 K). Most of the previous investigations were carried out at 283-288 K. A detailed study of the optimum leaching conditions and the kinetics of magnesia dissolution form the subject of the present investigation. MATERIALS AND METHODS This work was performed on a magnesite waste dump sample from Salem, Tamilnadu. The sample was investigated for the extraction of magnesia by the aqueous pressure carbonation process. The chemical and mineralogical analysis of the sample are shown in Table 1.
TABLE 1 Chemical and mineralogical analysis of Salem magnesite waste dump Chemical MgO SiO 2 Fez03 + A1203 CaO Mn S P LOI E
analysis
%
42.01 11.04 0.45 1.22 Trace 0.098 0.04 44.86 99.71%
Mineralogical Magnesite Quartz Dunite
(major),
analysis dunite
(minor)
Extraction of magnesia from low grade magnesites
81
The natural magnesite dump sample was predominantly MgCO3 with the silica mainly present as colloidal quartz filling inside the crevices of the magnesite crystals. Minor amounts of ferro-magnesium silicates were also found. The chemical and mineralogical studies revealed that the MgCO3 content to be about 84-85% and that very little magnesium was present as ferromagnesium silicate. The dump sample (-12+3 mm) was calcined at 973 K for 2 hours in an electrical muffle furnace, crushed and dry ground for 100% passing through 150#m for the leaching studies. High purity carbon dioxide of 99% obtained in cylinders was used in all the pressure carbonation reactions. EXPERIMENTAL All pressure leaching experiments were carried out in a (21) laboratory autoclave with interchangeable SS316 liners. The autoclave had a maximum working pressure of 60 kg/cm2, a temperature of 513 K and maximum stirring speed of 800 rpm. The temperature could be controlled to -+ 0.5 K of the required temperature. Low temperature tests were carried out by circulating cooled water (kept at predetermined temperature) through the cooling coils of the autoclave. The stirrer used resulted in good dispersion of CO2 in the pulp. CO2 of 99% purity was passed through an inlet port to the desired pressure and make up gas was periodically added whenever necessary. The sampling was performed via a SS316 capillary tube and a valve and samples were drawn at desired intervals. After each sampling, the sampling port was cleaned free of pulp by passing CO2 through the tube. The sampling was performed quickly and the samples were quickly filtered and analysed. In the procedure followed throughout the investigation the calcined magnesite sample, ground to different sizes (75 to 150#m, 44 to 75#m and below 44#m), was hydrated at ambient temperature to remove any calcium. The slurry of magnesium hydroxide was further hydrated under boiling conditions for 2 hours by refluxing and the cooled slurry along with make up water (added to adjust the pulp density) was transferred to the autoclave. After switching on the stirrer, 99% CO2 from a cylinder was passed through the slurry and the CO2 pressure was maintained at the desired level. Samples were drawn at various time intervals and make up COz was passed to maintain the CO 2 pressure. After the experiment was over, the contents of the reaction vessel were filtered. The residue was washed, dried, weighed and analysed for magnesium wherever necessary. All the magnesium analyses in the solutions were carried out by EDTA methods. The experiments were performed to study the effect of various parameters, viz: temperature, time, particle size, pulp density, carbon dioxide pressure, and magnesia calcined at various temperatures on magnesia dissolution. RESULTS AND DISCUSSION The magnesia extraction from calcined magnesite in the pressure carbonation process can be expressed by the following reactions: MgO + H20 = Mg(OH)2 (Hydration)
(1)
Mg(OH)2 + 2CO2 = Mg(HCO3)2 (Dissolution)
(2)
The solubility of MgO at 303 K is 2.1 g/100 g of CO 2 saturated solution at one atmosphere. It increases with decrease of temperature and decreases with increase of temperature. Figure 1 shows the magnesium dissolution as a function of pressure and time at ambient temperature (306 K). The pressures studied are 200 kPa, 390 kPa, 590 kPa and 790 kPa. The results show that the magnesium dissolution increased with increase of CO 2 pressure and time. The dissolution of magnesium is fast and reaches a maximum in 60 minutes or less. However the maximum concentration of magnesium in the resulting solution was constant
82
D. SHEILA et al.
over a range of COz pressures of 390 kPa to 790 kPa and is 9.164 g/l against an input (feed) of 9.168 g/l indicative of the dissolution of 99% of magnesium (all the concentrations are expressed in terms of Mg present as MgCO3 in original sample and Mg(OH)z in solution). Cielens and Konstants [18] who have studied the kinetics of dissolution of MgO at various partial pressures of CO 2 have found the carbonation rate to increase at first, pass through the maximum and then to gradually decrease to zero, indicative of no further dissolution. The present study follows the same trend. The optimum pressure chosen was 390 kPa. 100
1,0 0 0'9
gO
SO
X
R"
S/L
70
- ZOOMesh
ROOM T E M P E R A T U R E I 3 0 ~
CACLN. i
o.s
2011/I
SIZE
&O
X
),
0.7
-'4d')3K
0,6 ~'
2OO kPa ~4K) kPa
,o.,
50
O,S '~
590 kPa
0
7gOkPa
40
I--
O.4
3O
0,3
2O
O.Z
10
01
°o
,o ,o
;o ,'o
d, ~, ,6
~o ~o
~
~-
',,o ,~o ,~o ',,o ,~o
TIME(rain)
Fig.l Effect of pressure on the extraction of magnesium at ambient temperature (306K) The effect of particle size of the calcined magnesite on the dissolution of magnesium is shown in Fig.2. In these experiments different size fractions of calcine in the size range of 75 to 150/~m, 44 to 75#m and below 44#m were leached in CO 2 atmosphere with the following parameters held constant; CO2 pressure, 390 kPa; temperature 306 K, pulp density 2% solids, stirring speed 500 rpm, and magnesite calcined at 973 K. All the three size fractions showed similar effect on dissolution which is attributed to the hydration of calcined magnesite prior to carbonation. The Mg(OH) 2 particles have surface properties totally different from that of MgO. The Mg(OH) 2 particles being finer than MgO and hence the surface area of MgO had no critical effect on the rate of magnesium dissolution. IO0 90 J
8O 7O
t
• ZOgA : SSOkPa
PRESSURE
REACTION
TEMP. = 306 K
CACLN,
TEMP. = ~ 3 K
1'0 0'9
-O.O
0-7
O
6O
~r
SO
IO. 5
~.0
0.4
3O
0"3
2O
0"2
10
iO.I
0
0'6
10
t
t
:
~
20
30
&O
50
/
t
607000
~
t
t
J.
~
gO
100
I10
120
t
L30
t
140
i50
T I M E (rain)
Fig.2 Effect of particle size on extraction of magnesium
Extraction of magnesia from low grade magnesites
83
The effect of temperature on the extent of magnesium extraction has been studied over a temperature range from 286.5 K to 328 K and the results are shown in Figure 3. Magnesium extraction has been found to decrease with increase of temperature beyond the ambient temperature of 306 K. The decreased magnesium extraction at 328 K is due to low solubility of MgO. The highest rate and extent of magnesium extraction was realised at 286.5 K. At 286.5 K and 306 K the maximum magnesium concentration attained was 9.16 g/l against a feed of 9.19 g/l. In view of the limitations of carrying out experiments at lower temperatures of 286.5 K by refrigeration, further experiments were performed at 306 K as optimum.
100
I'0 A
9O
B
;.
-
O
80 70
~
6o
~
so
O
O'6
A
= 328K
Q
• $06K
O
= 286,5 K
0'7
S/L
= 209/I
PRESSURE
= 390 kPa
SIZE
= 200 Mesh
0'6 0"5
CACLN. TEMR= ~ J K
3O
0-9
O'4 -~ O'3
2O
t
O'2
I0
0.1
I 10
I 20
I 30
/0
I SO
I EO
I 70
1 9 1 0 1 80
100
I I10
I 120
I 130
I 140
150
TIME (rain)
Fig.3 Effect of temperature on extraction of magnesium Figures 4 and 5 represent the percentage of magnesium dissolution as a function of solid/liquid (S/L) ratio and time at 306 K and 286.5 K respectively. The CO2 pressure used in these experiments was 390 kPa. 100
1.0 0
90
O.9
80
0.8
70
O
13 S / I
60
O
209/I
50
A
25g/I
>( bJ
40
PRESSURE
: E g o kPo
SiZE
• 200 MeSh
"~
30
~<
0.7 0.6 0.5 i0'4
REACTION TEMR ' 3 0 t K CACLN. TEMP.
0"3
:~SK
2O 10
°o
t 0.2
0.1
,o
t
20
l
3o
l
,o
i
,o
A
T I M E (rain)
I
,o
I
,o
,~
I~o
,Io
,~,o ,~o
,~o
0
Fig.4 Effect of solid-liquid ratio on extraction of magnesium at 306K
D . SHEILA et al.
84
Figure 4 indicates that the magnesium dissolution decreased with increasing S/L ratios. For S/L ratio of 2.5 g/100 gram water, the precipitation of MgCO3.3HzO occurred prematurely resulting in the lowering of concentration of magnesium bicarbonate. Though the rate of magnesium dissolution was better at S/L ratio 1.3% solids, an optimum pulp density of 2% solids was chosen because of the resultant increased concentration of magnesium obtained in the leach liquor being 9.16 g/l against a theoretical maximum of 9.19 g/1. 100 90
JlO
0'11
z" / 70
o,oo,,,
f/
// ~
so
w
4,0
0.7
T..P
,o.,.
o
0"6 0.3
w &
S/L
0"4
2Og/I
I3O
0"3
2O
0"2
10
0'1
0
0
I 10
I 20
I 30
I 40
I SO
1 60
I 70
1 110
I 90
1 100
I 110
I 120
I
~tO
I~I0
160
TIME (rnin)
Fig.5 Effect of solid-liquid ratio on extraction of magnesium at 286.5K The effect of different S/L ratios on magnesium dissolution at 286.5 K is shown in Figure 5. For S/L ratio of 5% solids, the precipitation of MgCO3.3HeO was found to occur prematurely because of high magnesium concentration present in excess to the equivalent amount of CO2 absorbed. The S/L ratio of 3% (3 g/100 ml water) is optimum at 286.5 K. The resultant concentration of magnesium obtained in the leach liquor is 1.4 g/100 ml. Around the same temperature the theoretical solubility limit is about half the value (0.7 g/100 ml H20). The higher concentration at lower temperature observed is attributed to the meta stable state as explained by Evans and Hillary [14] and Belyaev [23]. The effect of stirring speed on leaching rate was investigated. The series of tests was conducted at 306 K, S/L ratio 2%, CO z pressure 390 kPa and for 60 minutes. The stirring speed was varied from 90 to 800 min "1. The stirring speed did not have much effect on the rate of dissolution. A stirring speed of 500 rpm was used in all experiments. The calcination temperature of magnesite plays an important role in the hydration and dissolution behaviour of magnesia. Magnesite calcined at low temperatures of 973-1073 K is quite reactive and is caustic whilst that calcined at temperatures in excess of 1173 K are less caustic and do not react well. Higher temperatures and longer calcination periods decrease the reactivity of magnesia. In order to study the effect of calcination temperature of magnesite on the leaching behaviour of calcine in CO2 saturated solutions, a series of tests were performed with the magnesite sample calcined at 973 K, 1023 K and 1073 K. Figure 6 represents the dissolution of magnesium at 306 K, S/L = 20 g calcine/litre and 390 kPa COe pressure. The dissolution of magnesium decreases with increasing calcination temperature. The optimum calcination temperature chosen was 973 K. The magnesium bicarbonate solutions have been analysed for impurities as well as the precipitated basic magnesium carbonate (BMC). The impurities in the basic magnesium carbonate were 0.08% CaO, 0.01% A120 3 plus Fe20 3 and 0.05% SIO2.
Extraction of magnesia from low grade magnesites
85
100
1.0 O.S
S/L
:20g/I O-S
8O
PRESSURE =390kPa
'°I
TEMP.
=306K
SIZE
:ZOOMes~
6O
D
=~3K
5O
O
,¢1023K
o-~
40
A
=1073 K
0'4
7O
0.7' O.S
l0
0.3
2O
O.Z
10
0.1
0
10
I 20
I 30
I 40
I SO
I SO
I 70
I 80
I 90
I 100
I IlO
l
120
I
[30
/ 140
F-
o
150
TIIdE(min)
Fig. 6 Effect of calcination temperature of magnesite on extraction of magnesium REACTION KINETICS Smithson and Bakshi [20] have delineated the reaction steps during the carbonation of MgO and have proposed the actual reaction mechanism using the Nsh value (Sherwood number) which may be expressed as: N s h = 2 + 0 . 6 ( N R e ) 1/2
(3)
(Nsc) 1/3
where Nsh = k c . d p / D v
(4)
where kc = Mass transfer coefficient (m/sec) dp = particle diameter cm Dv = diffusivity (cm2/sec) The mass transfer coefficient can be calculated from Rate = kc.a.A c
(5)
where a Ac
= surface area = concentration difference mol/litre
The present study involves interpretation of kinetic data in terms of reaction mechanism. The fraction transformed (~) Vs time curves (Fig. 1-6) were found to be linear up to 7080% of magnesium reacted after which it levelled off, characteristic of any rate controlling reaction dependent on the surface area of MgO such as film diffusion or surface chemical reaction. So the kinetic data has to be examined according to either film diffusion or chemical reaction. The following data were used to calculate Nsh values at 790 kPa.
86
D . SHE1LA
eta/.
Solubility of Mg(OH) 2 = 8.7x10 "3 g/cm3; Rate = 0.9/60.g/sec Surface area of Mg(OH) 2 = 1.2 x 20 x 104 cm 2 (BET method) ke
= 6.897 x 10 -5 cm/sec
dp
= 1.45 x 10 -6 cm 2 (X-ray diffraction)
Dv
= 2.05 x 10 -5 cm2/sec
Nsh
-- 6.897 x 10 -5 x 1.45 x 10 -6 / (2.05 x 10 -5 ) = 4.878 x 10 -6
The Nsh value being much less than 2, the possibility of film diffusion as rate controlling was ruled out and suggested a surface chemical reaction model. The experimental data were tested according to surface chemical reaction model using the equation: 1 - (1 - ~ ) 1 / 3 = k l t
(6)
but the plots were non linear. Smithson and Bakshi [20] who observed similar phenomena attributed this to the equation being derived for a single particle size whereas the Mg(OH)2 particles are composed of a range of particle sizes. They indicated the following equation-In (1 - a) = k2t
(7)
So all the kinetic data were analysed according to this model. The plot of -In (1 - cz) Vs t at different pressures (Fig. 7) is linear and the magnesium dissolution had a constant reaction rate indicating the validity of the model. The apparent activation energy determined from Arrhenius plot as shown in Figure 8 has been found to be 26.4 kJ mo1-1 (6.3 Kcal/mol-1). This energy of activation lies within the range reported for surface chemical reaction and is close to the value obtained by Smithson and Bakshi (7.2 Kcal mo1-1. The lower value being attributed to the prior hydration of MgO. Since the rate constants calculated from the experimental data fall below the minimum theoretical value for rate control by mass transfer, and because there is a reasonable fit of the rate data with the modified chemical reaction rate equation, it is concluded that the rate controlling step is a chemical reaction occurring at the surface of the Mg(OH)2 particles. 6 F"
o
i
;~00 kPl
X $10 kPa
5~ 590 kPa
/ 7 _
c I
3
/
0 7gO kPa
/ / X
1)0
lOgO
TIME ( m i n )
Fig. 7 Plot of -In ( l - a ) Vs time at different pressures
87
Extraction o f magnesia from low grade magnesites
In all the studies on dissolution of Mg(OH) 2, the initial pH of the slurry was 10.5 and decreased throughout the reaction to a pH of 7.46 as shown in Figure 9 at the completion of the experiment. This indicates that the hydration of CO2 is a minor reaction until near the end of carbonation of Mg(OH)2. Throughout the carbonation of Mg(OH) 2, the main reaction is between CO2(aq) and Mg(OH) 2 as reported earlier [20].
0"7 0'8 0"9 1"0
1"2 1.3
1.4 1.5 3"00 3"04 3"06 342
3"16 3"20 3"24 3"28 3Q2 3"36 3'L,0 3'~.
* 3%8 3"52 3'56 3~0
x 105(K "1 )
T
Fig.8 Plot of log k Vs 1/T x 103 (activation energy) Mg 11
2.Sl6S
S'7
1.97
CONCENTRATION I°$S
g/[
8-9S
!.11
9*lS
9-16/*
10
Z
O.
0 7 0
I
10
I
lO
I
30
I
~0
i SO TIME
I 60 (min)
e I 10
G l I0
I 90
t 100
I 110
Fig. 9 Variation of pH with time and magnesium concentration CONCLUSIONS The selective dissolution of magnesium from a low grade Salem magnesite sample can be achieved by pressure leaching with carbon dioxide. From the experimental studies it has been found that the dissolution of magnesium is enhanced by low reaction temperature and higher carbon dioxide pressure in the aqueous carbon dioxide leach process. The kinetic data were analysed in terms of modified chemical reaction rate equation and the apparent energy of activation of 26.4 KJ mol-1 suggested that the rate controlling step is chemical at the surface of the Mg(OH)2 particles.
88
D. SHEILA et al.
ACKNOWLEDGEMENT
The authors are thankful to Professor S. Banerjee, Director, National Metallurgical Laboratory, Jamshedpur for his encouragement and permission to publish this paper. REFERENCES
. . . .
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
K. Vijayaraghavan K., Satyanarayana C., Raman P.V. & Khangaonkar P.R., NML I.R.No. 863/76, Madras (1976). Vijayaraghavan K., Satyanarayana C., Mohan V. & Khangaonkar P.R., NML I.R.No. 1037/80, Madras (1980). Vijayaraghavan K., Satyanarayana C., Mohan V. & Khangaonkar P.R., NML I.R.No. 1203/85, Madras (1985). Kippe O., U.S. Patent 2912306, (1959). Grill M. & Bouvier G., British Patent 1332736, (1973). Cross H.E., Krieger W., Anschutz E. & Hirsh M., U.S. Patent No. 4096235, (1978). Ainscow W.S., Refractories Journal, 6-13 (May/June 1984). Mower Wm.W., Chemical Abstract, 47, 5647e (1953). Belyaev E.K. & Seryi B.G., Zh. Prikl. Khim, 51 (2), 472 (1978). Horiguchi Y., J. Sci. Research Inst., (Tokyo) 46, 258-267 (1952). Horiguchi Y., J. Sci. Research Inst., (Tokyo) 47,301-318 (1953). Gloss G.H., U.S. Patent 2 290095, (1945). Doerner H.A., Holbrook W.F. & Fortner O.W., U.S.Bur.Mines Tech. Paper 684, (1946). Evans R.L. & Hillery St. Clair W., Ind. Eng. Chem., 41 (12) (1949). Canterford J.H., Everson P.T. & Moyle F.J., Proc. Austras. Inst. Min. Metall. No. 277, 45-52 (March 1981). Shukla B.K. & Datar D.S., Ind. J. Appl. Chem., 34(3) (1971). Belyaev E.K., Tkach G.A., Tomenko V.M. & Seryi B.G., Otkrytiya Izobret, Prom. Obraztsy, Tovarnye Znaki, 54, (35), 76 (1977). Cielens U. & Konstants Z. et al., Latv. Psv. Zinat. Akad. Vertis Kim. Sci., 3, 285289 (Russian) (1969). Horiguchi Y., J. Sci. Res. Inst., 48, 27-37 (1954). Smithson G.L. & Bakshi N.N., Ind. Eng. Chem. Process Res. Dev., 12(1), 99-107 (1973). Sada E., Kumazawa H. & Lee C.H., Chem. Eng. Sci., 38(12), 2047-2051 (1983). Atherton Seidall, Solubilities of Inorganic and Metal Organic Compound, Vol I, 95. Belyaev E.K., Seryi B.G., Tomenko V.M. & Khomyakova V.V., Zh. Prikl Khim, 49, 8, 1892 (Russian) (1976).
0892-6875/91 $3.00 + IX) © 1990 Pergamon Press plc
Printed in Great Britain
STUDIES ON THE EXTRACTION OF MAGNESIA FROM LOW GRADE MAGNESITES BY CARBON DIOXIDE PRESSURE LEACHING OF HYDRATED MAGNESIA D. SHEILA§, C. SANKARANI and P.R. KHANGAONKARI
§ Centre for Water Resources, Guindy, Engineering College, Madras 60025, India t National Metallurgical Laboratory, Madras Centre, CSIR Madras Complex, India School of Material and Mineral Resources Engng., University Sains Malaysia, Perak Branch Campus, 30000 Ipoh, Malaysia (Received 6 February 1990; revision accepted 4 May 1990)
ABSTRACT
The applicability of carbon dioxide pressure leaching for the extraction of magnesia of high purity from low grade magnesites and waste dumps material has been investigated. Factors which influence the magnesia recovery such as time, temperature, pressure, particle size, solid~liquid ratio and calcination temperature have been studied and optimum values were determined. The kinetic data were examined according to therate equation -In(L- ~) against t and the activation energy of 26.4 kJ tool -1 (6.3Kcal tool "1) suggested that the kinetics o f the reaction are controlled by surface chemical reaction. Keywords Magnesite; carbon dioxide; hydration; pressure leaching; solubility; kinetics INTRODUCTION
Magnesite occurs extensively in Tamilnadu, Uttar Pradesh, Rajasthan, Kashmir and Karnataka in India. These natural magnesite deposits are contaminated with impurities like silica, iron oxides, alumina etc., and need to be suitably beneficiated prior to its use in refractories and in the preparation of magnesium compounds. Beneficiation of magnesite under Indian practice mostly consists of selective mining, screening and hand sorting to produce a marketable grade of magnesite with less than 4% SiO 2. Such a practice results in poor magnesite recoveries (30-35%) and substantial quantities are dumped. This has resulted in the piling up of huge magnesite dumps near the mine site. The rapid growth in the steel industry and particularly the increased use of tonnage oxygen in steel making has created heavy demands for high grade magnesite refractories. In order to cope with the increasing demand for high purity sintered magnesia, increasing attention is being paid throughout the world to the upgrading of low-grade and submarginal magnesites either by beneficiation or chemical treatment. The current supplies of high quality magnesia are derived from the processing of selected high grades of magnesite, low in SIO2, or from sea water. The basic oxygen converter and electric arc furnace steel making processes cannot accept low grade sintered magnesia as refractories and thus they depend heavily on sea water magnesia for their supplies. Beneficiation of magnesite would be the obvious solution to the need for alternative sources. 79
80
D. SHEILAet al.
Physical methods of beneficiation of magnesite [1-3] rely on the physical differences between magnesium carbonate and the impurities to effect the separation i.e. density, optical properties, gravity, flotation etc. Though these methods are economic, unfortunately they fail to achieve the desired results due to various limitations of the physical factors involved. Chemical beneficiation, relies on chemical interaction and selectivity of leaching of magnesia that results in its removal in a state of high purity. Many chemical methods are available to obtain magnesia of high purity from magnesite. The choice of a leachant depends on its selectivity and the nature of impurities. The use of common acids, like hydrochloric, sulphuric or nitric acids [4-6] has been examined, but has not been commercialised because of the complex purification and acid regeneration/recovery steps. A m m o n i u m chloride or calcium chloride leaching in the presence of carbon dioxide has been successfully used to selectively leach magnesia from calcined magnesite. The resulting magnesium chloride is treated with ammonia and carbon dioxide to precipitate magnesium carbonate which is then calcined, briquetted and sintered to produce high purity sintered magnesia. The calcium chloride leaching of the calcined magnesia in the presence of carbon dioxide forms the basis of the recently developed Sulmag (R) [7] process and all the reagents used at various stages in the process are recovered and suitably recycled. Carbon dioxide [8-21] has been found to selectively dissolve magnesia forming a soluble magnesium bicarbonate, leaving the impurities unaffected. The solution is decomposed by heating and/or aeration to form basic magnesium carbonate which is then suitably processed to obtain high purity magnesia. The major disadvantage of the carbon dioxide process is the relatively low solubility of magnesium bicarbonate resulting in increased capital costs when processing huge volumes of solutions for a given throughput of magnesia. High carbon dioxide pressures and low temperatures have been reported to increase the solubility of magnesium bicarbonate [22]. Despite the simplicity of the aqueous pressure carbonation process for the recovery of high purity magnesia, not much work has been done on Indian magnesites and waste dumps at the ambient temperatures (~303 K). Most of the previous investigations were carried out at 283-288 K. A detailed study of the optimum leaching conditions and the kinetics of magnesia dissolution form the subject of the present investigation. MATERIALS AND METHODS This work was performed on a magnesite waste dump sample from Salem, Tamilnadu. The sample was investigated for the extraction of magnesia by the aqueous pressure carbonation process. The chemical and mineralogical analysis of the sample are shown in Table 1.
TABLE 1 Chemical and mineralogical analysis of Salem magnesite waste dump Chemical MgO SiO 2 Fez03 + A1203 CaO Mn S P LOI E
analysis
%
42.01 11.04 0.45 1.22 Trace 0.098 0.04 44.86 99.71%
Mineralogical Magnesite Quartz Dunite
(major),
analysis dunite
(minor)
Extraction of magnesia from low grade magnesites
81
The natural magnesite dump sample was predominantly MgCO3 with the silica mainly present as colloidal quartz filling inside the crevices of the magnesite crystals. Minor amounts of ferro-magnesium silicates were also found. The chemical and mineralogical studies revealed that the MgCO3 content to be about 84-85% and that very little magnesium was present as ferromagnesium silicate. The dump sample (-12+3 mm) was calcined at 973 K for 2 hours in an electrical muffle furnace, crushed and dry ground for 100% passing through 150#m for the leaching studies. High purity carbon dioxide of 99% obtained in cylinders was used in all the pressure carbonation reactions. EXPERIMENTAL All pressure leaching experiments were carried out in a (21) laboratory autoclave with interchangeable SS316 liners. The autoclave had a maximum working pressure of 60 kg/cm2, a temperature of 513 K and maximum stirring speed of 800 rpm. The temperature could be controlled to -+ 0.5 K of the required temperature. Low temperature tests were carried out by circulating cooled water (kept at predetermined temperature) through the cooling coils of the autoclave. The stirrer used resulted in good dispersion of CO2 in the pulp. CO2 of 99% purity was passed through an inlet port to the desired pressure and make up gas was periodically added whenever necessary. The sampling was performed via a SS316 capillary tube and a valve and samples were drawn at desired intervals. After each sampling, the sampling port was cleaned free of pulp by passing CO2 through the tube. The sampling was performed quickly and the samples were quickly filtered and analysed. In the procedure followed throughout the investigation the calcined magnesite sample, ground to different sizes (75 to 150#m, 44 to 75#m and below 44#m), was hydrated at ambient temperature to remove any calcium. The slurry of magnesium hydroxide was further hydrated under boiling conditions for 2 hours by refluxing and the cooled slurry along with make up water (added to adjust the pulp density) was transferred to the autoclave. After switching on the stirrer, 99% CO2 from a cylinder was passed through the slurry and the CO2 pressure was maintained at the desired level. Samples were drawn at various time intervals and make up COz was passed to maintain the CO 2 pressure. After the experiment was over, the contents of the reaction vessel were filtered. The residue was washed, dried, weighed and analysed for magnesium wherever necessary. All the magnesium analyses in the solutions were carried out by EDTA methods. The experiments were performed to study the effect of various parameters, viz: temperature, time, particle size, pulp density, carbon dioxide pressure, and magnesia calcined at various temperatures on magnesia dissolution. RESULTS AND DISCUSSION The magnesia extraction from calcined magnesite in the pressure carbonation process can be expressed by the following reactions: MgO + H20 = Mg(OH)2 (Hydration)
(1)
Mg(OH)2 + 2CO2 = Mg(HCO3)2 (Dissolution)
(2)
The solubility of MgO at 303 K is 2.1 g/100 g of CO 2 saturated solution at one atmosphere. It increases with decrease of temperature and decreases with increase of temperature. Figure 1 shows the magnesium dissolution as a function of pressure and time at ambient temperature (306 K). The pressures studied are 200 kPa, 390 kPa, 590 kPa and 790 kPa. The results show that the magnesium dissolution increased with increase of CO 2 pressure and time. The dissolution of magnesium is fast and reaches a maximum in 60 minutes or less. However the maximum concentration of magnesium in the resulting solution was constant
82
D. SHEILA et al.
over a range of COz pressures of 390 kPa to 790 kPa and is 9.164 g/l against an input (feed) of 9.168 g/l indicative of the dissolution of 99% of magnesium (all the concentrations are expressed in terms of Mg present as MgCO3 in original sample and Mg(OH)z in solution). Cielens and Konstants [18] who have studied the kinetics of dissolution of MgO at various partial pressures of CO 2 have found the carbonation rate to increase at first, pass through the maximum and then to gradually decrease to zero, indicative of no further dissolution. The present study follows the same trend. The optimum pressure chosen was 390 kPa. 100
1,0 0 0'9
gO
SO
X
R"
S/L
70
- ZOOMesh
ROOM T E M P E R A T U R E I 3 0 ~
CACLN. i
o.s
2011/I
SIZE
&O
X
),
0.7
-'4d')3K
0,6 ~'
2OO kPa ~4K) kPa
,o.,
50
O,S '~
590 kPa
0
7gOkPa
40
I--
O.4
3O
0,3
2O
O.Z
10
01
°o
,o ,o
;o ,'o
d, ~, ,6
~o ~o
~
~-
',,o ,~o ,~o ',,o ,~o
TIME(rain)
Fig.l Effect of pressure on the extraction of magnesium at ambient temperature (306K) The effect of particle size of the calcined magnesite on the dissolution of magnesium is shown in Fig.2. In these experiments different size fractions of calcine in the size range of 75 to 150/~m, 44 to 75#m and below 44#m were leached in CO 2 atmosphere with the following parameters held constant; CO2 pressure, 390 kPa; temperature 306 K, pulp density 2% solids, stirring speed 500 rpm, and magnesite calcined at 973 K. All the three size fractions showed similar effect on dissolution which is attributed to the hydration of calcined magnesite prior to carbonation. The Mg(OH) 2 particles have surface properties totally different from that of MgO. The Mg(OH) 2 particles being finer than MgO and hence the surface area of MgO had no critical effect on the rate of magnesium dissolution. IO0 90 J
8O 7O
t
• ZOgA : SSOkPa
PRESSURE
REACTION
TEMP. = 306 K
CACLN,
TEMP. = ~ 3 K
1'0 0'9
-O.O
0-7
O
6O
~r
SO
IO. 5
~.0
0.4
3O
0"3
2O
0"2
10
iO.I
0
0'6
10
t
t
:
~
20
30
&O
50
/
t
607000
~
t
t
J.
~
gO
100
I10
120
t
L30
t
140
i50
T I M E (rain)
Fig.2 Effect of particle size on extraction of magnesium
Extraction of magnesia from low grade magnesites
83
The effect of temperature on the extent of magnesium extraction has been studied over a temperature range from 286.5 K to 328 K and the results are shown in Figure 3. Magnesium extraction has been found to decrease with increase of temperature beyond the ambient temperature of 306 K. The decreased magnesium extraction at 328 K is due to low solubility of MgO. The highest rate and extent of magnesium extraction was realised at 286.5 K. At 286.5 K and 306 K the maximum magnesium concentration attained was 9.16 g/l against a feed of 9.19 g/l. In view of the limitations of carrying out experiments at lower temperatures of 286.5 K by refrigeration, further experiments were performed at 306 K as optimum.
100
I'0 A
9O
B
;.
-
O
80 70
~
6o
~
so
O
O'6
A
= 328K
Q
• $06K
O
= 286,5 K
0'7
S/L
= 209/I
PRESSURE
= 390 kPa
SIZE
= 200 Mesh
0'6 0"5
CACLN. TEMR= ~ J K
3O
0-9
O'4 -~ O'3
2O
t
O'2
I0
0.1
I 10
I 20
I 30
/0
I SO
I EO
I 70
1 9 1 0 1 80
100
I I10
I 120
I 130
I 140
150
TIME (rain)
Fig.3 Effect of temperature on extraction of magnesium Figures 4 and 5 represent the percentage of magnesium dissolution as a function of solid/liquid (S/L) ratio and time at 306 K and 286.5 K respectively. The CO2 pressure used in these experiments was 390 kPa. 100
1.0 0
90
O.9
80
0.8
70
O
13 S / I
60
O
209/I
50
A
25g/I
>( bJ
40
PRESSURE
: E g o kPo
SiZE
• 200 MeSh
"~
30
~<
0.7 0.6 0.5 i0'4
REACTION TEMR ' 3 0 t K CACLN. TEMP.
0"3
:~SK
2O 10
°o
t 0.2
0.1
,o
t
20
l
3o
l
,o
i
,o
A
T I M E (rain)
I
,o
I
,o
,~
I~o
,Io
,~,o ,~o
,~o
0
Fig.4 Effect of solid-liquid ratio on extraction of magnesium at 306K
D . SHEILA et al.
84
Figure 4 indicates that the magnesium dissolution decreased with increasing S/L ratios. For S/L ratio of 2.5 g/100 gram water, the precipitation of MgCO3.3HzO occurred prematurely resulting in the lowering of concentration of magnesium bicarbonate. Though the rate of magnesium dissolution was better at S/L ratio 1.3% solids, an optimum pulp density of 2% solids was chosen because of the resultant increased concentration of magnesium obtained in the leach liquor being 9.16 g/l against a theoretical maximum of 9.19 g/1. 100 90
JlO
0'11
z" / 70
o,oo,,,
f/
// ~
so
w
4,0
0.7
T..P
,o.,.
o
0"6 0.3
w &
S/L
0"4
2Og/I
I3O
0"3
2O
0"2
10
0'1
0
0
I 10
I 20
I 30
I 40
I SO
1 60
I 70
1 110
I 90
1 100
I 110
I 120
I
~tO
I~I0
160
TIME (rnin)
Fig.5 Effect of solid-liquid ratio on extraction of magnesium at 286.5K The effect of different S/L ratios on magnesium dissolution at 286.5 K is shown in Figure 5. For S/L ratio of 5% solids, the precipitation of MgCO3.3HeO was found to occur prematurely because of high magnesium concentration present in excess to the equivalent amount of CO2 absorbed. The S/L ratio of 3% (3 g/100 ml water) is optimum at 286.5 K. The resultant concentration of magnesium obtained in the leach liquor is 1.4 g/100 ml. Around the same temperature the theoretical solubility limit is about half the value (0.7 g/100 ml H20). The higher concentration at lower temperature observed is attributed to the meta stable state as explained by Evans and Hillary [14] and Belyaev [23]. The effect of stirring speed on leaching rate was investigated. The series of tests was conducted at 306 K, S/L ratio 2%, CO z pressure 390 kPa and for 60 minutes. The stirring speed was varied from 90 to 800 min "1. The stirring speed did not have much effect on the rate of dissolution. A stirring speed of 500 rpm was used in all experiments. The calcination temperature of magnesite plays an important role in the hydration and dissolution behaviour of magnesia. Magnesite calcined at low temperatures of 973-1073 K is quite reactive and is caustic whilst that calcined at temperatures in excess of 1173 K are less caustic and do not react well. Higher temperatures and longer calcination periods decrease the reactivity of magnesia. In order to study the effect of calcination temperature of magnesite on the leaching behaviour of calcine in CO2 saturated solutions, a series of tests were performed with the magnesite sample calcined at 973 K, 1023 K and 1073 K. Figure 6 represents the dissolution of magnesium at 306 K, S/L = 20 g calcine/litre and 390 kPa COe pressure. The dissolution of magnesium decreases with increasing calcination temperature. The optimum calcination temperature chosen was 973 K. The magnesium bicarbonate solutions have been analysed for impurities as well as the precipitated basic magnesium carbonate (BMC). The impurities in the basic magnesium carbonate were 0.08% CaO, 0.01% A120 3 plus Fe20 3 and 0.05% SIO2.
Extraction of magnesia from low grade magnesites
85
100
1.0 O.S
S/L
:20g/I O-S
8O
PRESSURE =390kPa
'°I
TEMP.
=306K
SIZE
:ZOOMes~
6O
D
=~3K
5O
O
,¢1023K
o-~
40
A
=1073 K
0'4
7O
0.7' O.S
l0
0.3
2O
O.Z
10
0.1
0
10
I 20
I 30
I 40
I SO
I SO
I 70
I 80
I 90
I 100
I IlO
l
120
I
[30
/ 140
F-
o
150
TIIdE(min)
Fig. 6 Effect of calcination temperature of magnesite on extraction of magnesium REACTION KINETICS Smithson and Bakshi [20] have delineated the reaction steps during the carbonation of MgO and have proposed the actual reaction mechanism using the Nsh value (Sherwood number) which may be expressed as: N s h = 2 + 0 . 6 ( N R e ) 1/2
(3)
(Nsc) 1/3
where Nsh = k c . d p / D v
(4)
where kc = Mass transfer coefficient (m/sec) dp = particle diameter cm Dv = diffusivity (cm2/sec) The mass transfer coefficient can be calculated from Rate = kc.a.A c
(5)
where a Ac
= surface area = concentration difference mol/litre
The present study involves interpretation of kinetic data in terms of reaction mechanism. The fraction transformed (~) Vs time curves (Fig. 1-6) were found to be linear up to 7080% of magnesium reacted after which it levelled off, characteristic of any rate controlling reaction dependent on the surface area of MgO such as film diffusion or surface chemical reaction. So the kinetic data has to be examined according to either film diffusion or chemical reaction. The following data were used to calculate Nsh values at 790 kPa.
86
D . SHE1LA
eta/.
Solubility of Mg(OH) 2 = 8.7x10 "3 g/cm3; Rate = 0.9/60.g/sec Surface area of Mg(OH) 2 = 1.2 x 20 x 104 cm 2 (BET method) ke
= 6.897 x 10 -5 cm/sec
dp
= 1.45 x 10 -6 cm 2 (X-ray diffraction)
Dv
= 2.05 x 10 -5 cm2/sec
Nsh
-- 6.897 x 10 -5 x 1.45 x 10 -6 / (2.05 x 10 -5 ) = 4.878 x 10 -6
The Nsh value being much less than 2, the possibility of film diffusion as rate controlling was ruled out and suggested a surface chemical reaction model. The experimental data were tested according to surface chemical reaction model using the equation: 1 - (1 - ~ ) 1 / 3 = k l t
(6)
but the plots were non linear. Smithson and Bakshi [20] who observed similar phenomena attributed this to the equation being derived for a single particle size whereas the Mg(OH)2 particles are composed of a range of particle sizes. They indicated the following equation-In (1 - a) = k2t
(7)
So all the kinetic data were analysed according to this model. The plot of -In (1 - cz) Vs t at different pressures (Fig. 7) is linear and the magnesium dissolution had a constant reaction rate indicating the validity of the model. The apparent activation energy determined from Arrhenius plot as shown in Figure 8 has been found to be 26.4 kJ mo1-1 (6.3 Kcal/mol-1). This energy of activation lies within the range reported for surface chemical reaction and is close to the value obtained by Smithson and Bakshi (7.2 Kcal mo1-1. The lower value being attributed to the prior hydration of MgO. Since the rate constants calculated from the experimental data fall below the minimum theoretical value for rate control by mass transfer, and because there is a reasonable fit of the rate data with the modified chemical reaction rate equation, it is concluded that the rate controlling step is a chemical reaction occurring at the surface of the Mg(OH)2 particles. 6 F"
o
i
;~00 kPl
X $10 kPa
5~ 590 kPa
/ 7 _
c I
3
/
0 7gO kPa
/ / X
1)0
lOgO
TIME ( m i n )
Fig. 7 Plot of -In ( l - a ) Vs time at different pressures
87
Extraction o f magnesia from low grade magnesites
In all the studies on dissolution of Mg(OH) 2, the initial pH of the slurry was 10.5 and decreased throughout the reaction to a pH of 7.46 as shown in Figure 9 at the completion of the experiment. This indicates that the hydration of CO2 is a minor reaction until near the end of carbonation of Mg(OH)2. Throughout the carbonation of Mg(OH) 2, the main reaction is between CO2(aq) and Mg(OH) 2 as reported earlier [20].
0"7 0'8 0"9 1"0
1"2 1.3
1.4 1.5 3"00 3"04 3"06 342
3"16 3"20 3"24 3"28 3Q2 3"36 3'L,0 3'~.
* 3%8 3"52 3'56 3~0
x 105(K "1 )
T
Fig.8 Plot of log k Vs 1/T x 103 (activation energy) Mg 11
2.Sl6S
S'7
1.97
CONCENTRATION I°$S
g/[
8-9S
!.11
9*lS
9-16/*
10
Z
O.
0 7 0
I
10
I
lO
I
30
I
~0
i SO TIME
I 60 (min)
e I 10
G l I0
I 90
t 100
I 110
Fig. 9 Variation of pH with time and magnesium concentration CONCLUSIONS The selective dissolution of magnesium from a low grade Salem magnesite sample can be achieved by pressure leaching with carbon dioxide. From the experimental studies it has been found that the dissolution of magnesium is enhanced by low reaction temperature and higher carbon dioxide pressure in the aqueous carbon dioxide leach process. The kinetic data were analysed in terms of modified chemical reaction rate equation and the apparent energy of activation of 26.4 KJ mol-1 suggested that the rate controlling step is chemical at the surface of the Mg(OH)2 particles.
88
D. SHEILA et al.
ACKNOWLEDGEMENT
The authors are thankful to Professor S. Banerjee, Director, National Metallurgical Laboratory, Jamshedpur for his encouragement and permission to publish this paper. REFERENCES
. . . .
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
K. Vijayaraghavan K., Satyanarayana C., Raman P.V. & Khangaonkar P.R., NML I.R.No. 863/76, Madras (1976). Vijayaraghavan K., Satyanarayana C., Mohan V. & Khangaonkar P.R., NML I.R.No. 1037/80, Madras (1980). Vijayaraghavan K., Satyanarayana C., Mohan V. & Khangaonkar P.R., NML I.R.No. 1203/85, Madras (1985). Kippe O., U.S. Patent 2912306, (1959). Grill M. & Bouvier G., British Patent 1332736, (1973). Cross H.E., Krieger W., Anschutz E. & Hirsh M., U.S. Patent No. 4096235, (1978). Ainscow W.S., Refractories Journal, 6-13 (May/June 1984). Mower Wm.W., Chemical Abstract, 47, 5647e (1953). Belyaev E.K. & Seryi B.G., Zh. Prikl. Khim, 51 (2), 472 (1978). Horiguchi Y., J. Sci. Research Inst., (Tokyo) 46, 258-267 (1952). Horiguchi Y., J. Sci. Research Inst., (Tokyo) 47,301-318 (1953). Gloss G.H., U.S. Patent 2 290095, (1945). Doerner H.A., Holbrook W.F. & Fortner O.W., U.S.Bur.Mines Tech. Paper 684, (1946). Evans R.L. & Hillery St. Clair W., Ind. Eng. Chem., 41 (12) (1949). Canterford J.H., Everson P.T. & Moyle F.J., Proc. Austras. Inst. Min. Metall. No. 277, 45-52 (March 1981). Shukla B.K. & Datar D.S., Ind. J. Appl. Chem., 34(3) (1971). Belyaev E.K., Tkach G.A., Tomenko V.M. & Seryi B.G., Otkrytiya Izobret, Prom. Obraztsy, Tovarnye Znaki, 54, (35), 76 (1977). Cielens U. & Konstants Z. et al., Latv. Psv. Zinat. Akad. Vertis Kim. Sci., 3, 285289 (Russian) (1969). Horiguchi Y., J. Sci. Res. Inst., 48, 27-37 (1954). Smithson G.L. & Bakshi N.N., Ind. Eng. Chem. Process Res. Dev., 12(1), 99-107 (1973). Sada E., Kumazawa H. & Lee C.H., Chem. Eng. Sci., 38(12), 2047-2051 (1983). Atherton Seidall, Solubilities of Inorganic and Metal Organic Compound, Vol I, 95. Belyaev E.K., Seryi B.G., Tomenko V.M. & Khomyakova V.V., Zh. Prikl Khim, 49, 8, 1892 (Russian) (1976).