Soda ash leaching of scheelite concentrates: the effect of high concentration of sodium carbonate

hydrometallurgy

~!~11,1,3 . ' ~)'.? ELSEVIER

Hydrometallurgy 46 (1997) 191-203

Soda ash leaching of scheelite concentrates: the effect of high concentration of sodium carbonate J.P. Martins *, F. Martins Departamento de Engenharia QMmica, Laboratdrio de Catdlise e Materiais, FEUP, Rua dos Bragas, 4099. Porto, Portugal

Received 2 October 1996; accepted 10 February 1997

Abstract

The effect of high concentrations of sodium carbonate in soda ash digestion of scheelite concentrates has been studied by analysing the temperature profile through the calcium carbonate ash layer, and using a calcium sequestering agent (EDTA) in the leaching system. A mechanism based on the principles of the adsorption phenomena involved is described to justify the probable presence of a double compound of sodium and calcium carbonates blocking the calcite pores.

1. I n t r o d u c t i o n The use of soda ash leaching as a complementary procedure to the dressing techniques applied to tungsten ores is a challenge to researchers looking for an improvement in the recovery of tungsten from low-grade scheelite concentrates ( 5 - 2 0 wt% WO3). A first discussion of this topic was given by Hamilton [1], and Maslenitskii [2] took into account the reaction of scheelite with sodium carbonate: CaWO4(s ) + NazCO3(aq ) ¢* NazWOa(aq) + CaCO3(s )

(1)

which at 25°C has a standard enthalpy of 37.6 k J / m o l and a free energy of 56.5 k J / m o l [3]. This means that it is necessary to operate at high temperatures (above 100°C). The removal of about 9 0 - 1 0 0 wt% of the tungsten initially in the particles requires a 2 0 0 - 3 0 0 % NaeCO 3 excess over the stoichiometric amount and a temperature of 200-250°C [2-5]. There is some controversy about the step controlling the reaction rate.

* Corresponding author. 0304-386X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI1 S0304-386X(97)00013-3

192

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203

According to Belikov and Maslenitskii [6] and Queneau [7] the reaction is under diffusion control, while the chemical regime was assumed by Zelikman and Meerson [8]. It seems reasonable to consider that, after a short reaction time, the ash layer of calcium carbonate associated with the structure of the solid particles plays an important role in leaching [3]. It is possible to increase the reaction rate, and thereby reduce the leaching time, either by the use of finely ground concentrates or with an operational system that disaggregates the CaCO 3 around the solid particles, such as a mechanical effect [9] or elastic vibrations [10-13]. Since the ore dissolution is a heterogeneous reaction, a reduction in the leaching time with the increase of the leaching concentration is also expected, as a consequence of the Le Ch~telier principle. However, Perlov [ 14] observed a decrease in tungsten conversion with soda ash concentration of 230 g / l at high temperature. This behaviour was attributed by Perlov to the formation of a complex compound of Na2CO 3 - CaCO 3 and Na2CO 3 - 2CaCO 3. The precipitation of this double salt not only reduces the amount of reagent in solution but also reduces its diffusion rate to the unreacted core of scheelite. Belikov and Maslenitskii [6] agreed with this behaviour but they claim that it starts at a lower concentration (150 g / l Na2CO3). Nasu [15] explains this negative effect with a corresponding increase of the solubility of CaCO 3 in Na2CO 3 solution. He reported the following solubility data for calcium carbonate at 197°C: 0.3 g/1 in a 50 g / l Na2CO 3 solution, 0.7 g / l at 150 g/1 Na2CO 3 and 1.2 g / l at 250 g/1 Na2CO 3. As CaCO 3 is a reaction product, its solubility shifts the equilibrium of reaction (1) to the left. Without sacrificing the tungsten conversion at high Na2CO 3 concentrations, Queneau et al. [16] suggest that a better control of the reaction can be obtained during leaching by adding the Na2CO 3 in small increments, thus preventing formation of the complex compound. In this way the soda ash process can be extended to refined scheelite without the need to use low solid to liquid ratios. This paper intends to contribute to a better insight into the high concentration soda ash digestion, by studying the temperature profile through the ash layer and performing the leaching in the presence of a calcium sequestering agent.

2. Non-catalytic solid-fluid model Solid-fluid reactions are an important field in chemical and metallurgical industries that includes the combustion of solid fuels, the reduction of metallic oxides, the roasting and leaching of ores, etc. When the reaction occurs on a non-porous solid particle, the analysis of the rate controlling factors are normally based on the shrinking core model (SCM) [17] or on the shrinking model (SM) [18]. In a previous study of soda ash leaching of low-grade scheelite concentrates it was found that the SCM describes the kinetics of the process [3].

2.1. Conversion expressions for the shrinking core model Let us assume the following: an irreversible chemical reaction, first order in the leaching agent and zero order in the solid; a constant particle size and the pseudo-

J.P. Martins, F. Martins/ Hydrometallurgy46 (1997) 191-203

193

steady-state. After manipulation, the material and energy balance equations within the inert ash layer around the particles can be written as: CAc

1 I + D 2 D,

+

T~- 1 7, -- -- 0204 I"~AI) 03

" 1--~o

+

.

exp D 5 l - - ~ - -

1_

[

exp 05 l _

(3)

Simultaneous solution of Eqs. (2) and (3) determines Cac and Tc. The next step is to relate the shrinking of the core with the rate of the surface reaction and hence with CAc. According to the stoichiometry of the reaction of scheelite with sodium carbonate, the relationship between the leaching time and the position of the reactive phase is:

kIMBCAIt

rsRo

=

(

l-

+

r~][1 D2[ r~ + l+--Ro]~ ~ Ro 1+

+

2(rc]2] exp [ D 5(Tl) ] 1[Ro]

exp D 5 1 - - ~ -

(4)

The relation between the reacting core radius and the particle radius is given by:

X=l-

(,13 Ro

(5)

Substituting this relation in Eq. (4), we can express the evolution of the solid particle conversion with time.

2.2. Discussion The influence of the dimensionless parameters Dj to D 5 on the irreversible reaction is shown through the concentration and temperature profiles in the unreacted core. We will focus mainly on the temperature profile, since we want to analyse the solubility of the sodium carbonate at the reaction surface. Fig. 1 shows concentration and temperature profiles inside the particle ash layer for given values of DI-D 4 and three different values of the Arrhenius number /)5; it is seen that, as D 5 decreases, the temperature gradient between the bulk and reaction front increases. A similar behaviour is obtained with the increase in the Prater thermicity factor D4, as illustrated in Fig. 2. In this case, the cause of the temperature drop is the ash layer low thermal conductivity. An increase in the temperature drop, more accentuated at the beginning of the reaction, is also observed in the following situations: (1) an increase in the ash layer diffusion resistance relative to the resistance to film fluid diffusion; that is, the reciprocal of the mass Biot number, Dj, decreases, as shown in Fig. 3; (2) a decrease in the resistance to heat

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203

194

o.91"°z,,~%.t,:,::...=.

--~ . . . . . . . . . . . . . . . . . .

"":"-.

" ......

=.

.

.

.

.

.

.

"ii

--I-

~

o.6

.....

~ = ~ V 0.3 ~

,¢

~0,4

/

.

.....

.....

X

.....

;

.-.

CAe/CAI (DS=2.0E1) +CAc/CAI (DS=5.0E0) -- u - - Te/TI (D5=I.0EI)

/

.X, - - TdT1 (D5=2.0EI) - - o - - Te/TI (D5=5.0E0)

- -

0.2

0.1

Du=I0 -~, I)2=102, 1)3=10 2, D+= 10-2 Ii ...

0.0

. . . . .

0.0

L

0.2

. . . . . . . . . . . . .

0.4

= .......

0.6

0.8

1.0

rJl~ Fig. 1. R e l a t i o n b e t w e e n the n o r m a l i s e d c o n c e n t r a t i o n ( C A c / CAI ) or n o r m a l i s e d t e m p e r a t u r e ( T c / T 1) w i t h the n o r m a l i s e d p o s i t i o n at the u n r e a c t e d c o r e ( r c / R o ) : i n f l u e n c e o f A r r h e n i u s n u m b e r ( D s ) .

. . . . . .x. .

I".,

o.o+

.,.

0.8 +

._.----

r"i :

""'m.

0.7 i

.... • .

.

.

.

.

.

: -IS

~

0.6T

r.)

0.4

i'

'

0.3 0.2 0.1 0.0 0.0

0.2

i ................ 0.6 0.8

0.4

1.0

rJ1L+

Fig. 2. Relation between the normalised concentration (CAc /CAI) or normalised temperature (Tc /T~) with the normalised position at the unreacted core (r c/Ro): influence of Prater thermicity factor (D4).

195

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203 ~.o ~ .

.

0.9 ~

' ' ill..

'I

.

.

.

.

.

"" ' " " I - . .

0.8 0.7

j ~

iI(

*

0.3

~

li,,/

~

0.2 i

-rcrrl (I)I= I.OtO)

,

o~-2).._,

D2=]0z, D~=102,D~=I0"2, Ds=I0I

0.1 0.0

1

~

0.0

0.2

'

0.4

0.6

0.8

1.0

rJRo Fig. 3. Relation between the normalised concentration (CAc /CAI) or normalised temperature (Tc / T I) with the normalised position at the unreacted core ( r c / R o ) : influence of reciprocal mass Biot number (Di).

1.0 .~11:. . . . . x - - : - z : ~ x - ; ; ~ : i

0.9 r !

x,:-::.x..:-.~X

....

;oX I . . . .

.x-.

x"

",,

" " "'

"'= ",

-.;~'~

i'!

°.

J*'"-

'

~(-,

L:i..I

"'ll.

0.8 i

2

""J, .... • .

.

.

.

•

0.7

~

0.5

~'~ 0"6 T ~-

~

•

0.4 0.3 0.2 0.1 0.0 ! 0.0

0,2

b

~

0.4

0.6 rc/R

................ 0.8

~ -1.0

u

Fig. 4. Relation between the normalised concentration ( C A c / CA=) or normalised temperature ( T c / T ] ) with the normalised position at the unreacted core ( r e / R o ) : influence o f reciprocal heat Blot number (D3).

196

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203

0.9 0.8 0.7 -

~

0.6

0.5

~ 0.4 0.3 0.2 0.1 0.0 0.0

0.2

0.4

0.6

0.8

1.0

]'~,

Fig. 5. Relation between the normalised concentration (CAc / CAI) or normalised temperature (Tc / TI) with the normalised position at the unreacted core (r c/Ro): influence of Thiele modulus squared (D2).

transfer conduction relative to the resistance to convective heat transfer; that is, the reciprocal of the heat Biot number, D 3, increases, as shown in Fig. 4; and (3) an increase in the ash layer diffusion resistance relative to the resistance to chemical reaction; that is, the Thiele modulus squared, D 2, increases, as shown in Fig. 5.

3. Experimental details

3.1. Characterization of scheelite A medium-grade concentrate was supplied by Minas de Tarouca. Chemical analysis by atomic absorption spectroscopy and gravimetric methods, mineralogical composition by X-ray power diffraction pattern and sieve analysis are shown in Tables 1 and 2.

3.2. Leaching experiments The leaching tests at atmospheric pressure were carried out at 100 ___I°C and under pressure in a 1 1 Parr model 4531 autoclave, using 40-80 g of sample. The timing of the experiment was begun only after the required temperature was attained. At the end of each run the heating was stopped and the autoclave allowed to cool to 50°C. Reagent grade Na2CO 3 and Na2H2EDTA were used throughout in the experiments. At the end of each experiment the mixture was filtered and the residues washed, using about twice the volume of the filtrate. Aliquots were collected and analysed for

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203

197

Table 1 Chemical analysis and mineralogical composition of the medium-grade scheelite concentrate Chemical analysis

Mineralogical composition

Chemical constituent

Content (%)

Mineral

Content (%)

WO 3 F% 03 CaO SiO2 TiO2 S A1203 SnO~

40.64 18.76 17.30 10.65 5.79 3.76 2.16 1.00

scheelite garnet ilmenite hematite pyrite pyrrhotyte quartz ferberite cassiterite

49.50 19.00 11.00 8.50 5.00 3.00 2.00 1.00 1.00

Table 2 Sieve analysis of three lots of medium-grade scheelite concentrate Size (Ixm)

Fraction (%)

149 88 74 63 < 63

10 23 13 13 41

tungsten by the cinchonin method; care was taken in order to avoid the escape of tungsten to the washing solutions and a spot-test with stannous chloride was used in order to verify that this requirement was fulfilled. The leached solids, after drying, were characterised by X-ray diffraction.

4. Results and discussion To approximately calculate the values of the former SCM dimensionless parameters, the following information is used: the Belikov and Maslenitskii [6] and Martins [3] kinetic data; the Ranz and Marshall correlation for heat transfer coefficient [19]; the R o w e and Claxton correlation for mass transfer coefficient [20]; an estimated diffusivity coefficient ranging from 10 -11 m 2 / s to 10 -9 m 2 / s , typical for liquids. Thus we get the following results for the temperature range between 100°C and 200°C: D1 = 1 0 - 4_ 10- 2 ; D e = 1 0 - 4 - 1 0 - 2 ; D 3 = l - 2 ; D 4 --- 1 0 - 6 - 1 0 - 4 ; D 5 --- 11-14. The solubility of monohydrate sodium carbonate changes with temperature from 505 g / 1 (30°C) to 455 g / 1 (100°C), while the decahydrate salt varies from 70 g / 1 (0°C) to 388 g / 1 (100°C) [21]. From Figs. 1 - 5 it is obvious that, for those soda ash system dimensionless parameter

J.P. Martinx, F. Martins / Hydrometallurgy 46 (1997) 191-203

198 70 m _

] 60

5O

X

~ 40

")+6

1

30z-eo= 20i O 10

i

0

i 0

1

--!

i

t

i

i

--

. . . . . .

.... - ~t~i~g=Tsomi." -T . . . .

i

solid/liquid weight ratio~l/5

i

I

2

4

3

5

6

7

8

time (h) Influence of the leaching composition on the WO 3 recovery at 100°C; e] = stoichiometric relation Na 2CO 3 / W O 3; e 2 = stoichiometric relation H 2 Na2 E D T A / W O 3 - K e y : Curve pH e1 e2 W O 3 ore content ( % ) F i g . 6.

1

11.9

5

-

40.64

2

11.6

2

-

40.64

3

10.4

5

1

40.64

4

9.4

2

1

40.64

5

11.9

5

-

5.80

6

9.4

-

1

40.64

values, a temperature drop enough to prevent the solubilization of the leaching agent cannot be attained. The effect of a high concentration of sodium carbonate, with or without H 2 N a 2 E D T A , on the leaching conversion at the temperature of 100°C is shown in Fig. 6. As can be seen from this figure, the dissolution of medium-grade scheelite concentrate (40.64 wt% WO 3) with sodium carbonate is the same whether using the concentration of 74.3 g / l (e I = 2) or 185.8 g / l (e I = 5). However, if 130 g/1 (e 2 = 1) of H 2 N a 2 E D T A is used with either of these, there is better tungsten extraction for the lower concentration. In the last case the sum of the conversions for each one of the leaching runs with Na2CO 3 or H 2 N a 2 E D T A , respectively (curves 2 and 6 in Fig. 6) is lower than the extraction with both reagents. This synergism is the result of the chemical reactions occurring in the system. As a matter of fact, the following reactions occur simultaneously with reaction

(1): CaWO4(s) + y 4 - (an) ~ Cay2- (aq) + WO42- (aq)

(6)

CaCO3(s) + y 4 - (aq) ~ CaV 2- (aq) + CO 2- (aq)

(7)

where y 4 - represents the tetra-anion of E D T A in solution. The rate constant at 100°C for reaction (1) is 4 × 10 - 9 m / / s , according Martins [3], while the calculated value for

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203

~

.........~ - -

70

"~

!

|

~

/ /!/

o

!

;

1

.

2

.

=

.

.

.

3

.

.

.

.

4

.

5

.

,

,

stirring=750 min-'

~'""""'~i

_

0

te,~s:T20o~c

'

I

199

i

.

6

7

time (h) Fig. 7. Effect of the leaching composition and temperature on the WO 3 recovery from the medium-grade scheelite concentrate (40.64 wt% WO3).

reaction (2), using the experimental results of Konishi et al. [22], is 4.6 × 10 -9 m / s . Hence, it is reasonable to accept a rate constant of the same order for reaction (3). So, the decrease in the coating of calcium carbonate justifies the observed synergism. The influence of temperature on the leaching for the highest concentration of Na2CO 3 (e, = 5), with or without HzNa2EDTA (e 2 = 1), is shown in Fig. 7. It can be seen from this figure that the usefulness of the sequestering agent decreases when the temperature increases. The Arrhenius plot constructed for the first 30 min of the reaction with Na2CO 3 and H2Na2EDTA, shown in Fig. 8, gives an apparent activation energy of 30 k J / m o l . This value indicates a mixed control of the reaction that conforms with the 10.5

10 9.5 9 8.5

7.5 2.1

2.2

2.3

2.4

2.5

2.6

2.7

10"~x(l/T) (K q ) Fig. 8. Arrhenius plot for the dissolution of medium-grade scheelite concentrate with Na2CO 3 (e 1 = 5) and HzNa2EDTA (e 2 = 1) for the first 30 rain.

200

J.P. Martins, F. Martins / HydrometaUurgy 46 (1997) 191-203

(a) =

I,,°°1,.

21

26

(b)

~

~ 20---~

34

~36

.~

r,#/

¢

"....~ Ill

I.|l*

26

I*

I I* Ii

Ii b*lilllil

21

i|lllllil|,l.*

31

3~ 2 e "-'~

|llJ.I

....

I ....

I ,ll,I

,.*,1

....

1

34

Fig. 9. X - r a y p o w e r diffraction pattern (single-crystal m o n o c h r o m a t i z e d C u K ot radiation). (a) G r a i n after l e a c h i n g u n d e r the f o l l o w i n g conditions: T = 100°C, t = 15 min, e I = 5, e 2 = 1, s o l i d / l i q u i d w e i g h t ratio = 1 / 5 , stirring rate = 7 5 0 min -1 . (b) G r a i n after l e a c h i n g u n d e r the f o l l o w i n g conditions: T = 100°C, t = 4 h, e~ = 5, s o l i d / l i q u i d w e i g h t ratio = 1 / 5 , stirring rate = 7 5 0 min 1.

presence of calcite. The rate constant for all the reactions is of the same order of magnitude, but the sodium carbonate concentration is much higher than the concentration of H2Na2EDTA. This means that the production of an ash layer is more efficient in the reaction rate decrease in the final leaching times, when the concentration of free H2Na2EDTA is lower. In Fig. 9 the analysis of the X-ray diffraction pattern of the external surface of grains after leaching is shown. For the first 15 min with Na2CO 3 (e 1 = 5) and H2Na2EDTA (e 2 = 1) no calcite layer is detected (Fig. 9a); conversely, when using only the sodium carbonate, the calcite layer is clearly detected, since the characteristic peak of Na2CO 3 • 10H20 is observed (Fig. 9b). This behaviour agrees with the conclusions of Perlov [14] about a double salt precipitated on the unreacted core of the scheelite particles. It is our belief that the mechanism responsible for this precipitation is as follows: 1. The NazCO 3 attacks the surface of scheelite particles leading to a CaCO 3 coating. The reaction front moves steadily into the particle.

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203

201

2. During the diffusion of CO32- through the pores of the ash layer there is an adsorption of the anion on the calcite. The neutrality condition for the system consequently also imposes an increase in the local concentration of sodium ion. 3. The solubility limit for the sodium carbonate is then reached in those micro-volumes, and the resulting precipitate blocks the pores leading to dominant diffusion control. Increasing temperature decreases the blocking effect, since the adsorption of CO 3on the calcite surface is reduced and, consequently, the solubility limit of sodium carbonate is not attained. This conforms with the normal behaviour of adsorption, which is an exothermic phenomenon. It is worth mentioning that, since the solubility of the monohydrate decreases with temperature [21], an explanation based on the increase of solubility with temperature is not satisfactory.

5. Conclusions A high soda ash concentration imposes a decrease in the leaching conversion of scheelite concentrates. This seems to be a consequence of sodium carbonate precipitation, as a result of the interaction of the carbonate ion with the calcite layer. By increasing temperature or using a sequestering calcium agent, such as HzNa2EDTA, such an effect can be reduced. It is concluded that, for small particles, the temperature drop through the ash layer alone cannot justify the precipitation of the leaching agent. A mechanism based on the adsorption of the carbonate anion is proposed to describe the blocking effect of the precipitated salt.

6. List of symbols C D 1 = DeA/kte

o

D z = k IR o / D ~ A D 3 = kJhR

o

D4 = (AH)CAIDeA/TIk

D5 = E/RT, DeA e1 e2

E AH h k

ik e

e

fluid concentration (kmol/m 3) reciprocal mass Biot number Thiele modulus number squared reciprocal heat Biot number Prater thermicity factor Arrhenius number effective diffusivity of fluid (m2/s) stoichiometric relation NazCO3/WO 3 (kmol/kmol) stoichiometric relation H z N a 2 E D T A / W O 3 (kmol/kmol) activation energy (kJ/kmol) reaction-enthalpy change of (kJ/kmol) heat transfer coefficient between fluid and solid particle (kJ/m2sK) specific reaction rate for surface reaction at r c (m/s); k I = k evaluated at T = 7"1; k c = k evaluated at Tc effective thermal conductivity of ash layer (kJ/msK)

202

kf M r g o

R t T X

J.P. Martins, F. Martins / Hydrometallurgy 46 (1997) 191-203 mass transfer c o e f f i c i e n t b e t w e e n fluid and solid particle (m/s) molecular weight radius variable within ash layer (m); r c = radius o f unreacted core initial radius o f spherical particle (m) gas constant reaction time (s) temperature (K) fractional d e g r e e o f c o n v e r s i o n

Subscrip~ aq A B c 1 s

a q u e o u s solution liquid reactant solid reactant surface o f unreacted core; r e a c t i v e plane bulk liquid reactant solid particle; interface b e t w e e n liquid and solid

References [1] E.M. Hamilton, U.S. Pat. 1,261,383 (1918). [2] I.N. Maslenitskii, Autoclave process of tungsten extraction from its concentrates, Tsvet. Metal. 4-5 (1939) 140-143. [3] J.P. Martins, Kinetics of soda ash leaching of low-grade scheelite concentrates, Hydrometallurgy 42 (2) (1996) 221-236. [4] N.N. Maslenitskii, Soda process for treatment scheelite concentrates, Obogashch. Rud. 2 (4) (1957) 3-10. [5] N.N. Maslenitskii, P.M. Perlov, Development of the autoclave process for the treatment of tungsten concentrates. Proc. Int. Mineral Processing Congr. Group 111, Pap. 41, London, 1960. [6] V.V. Belikov, I.N. Maslenitskii, Kinetics of soda leaching scheelite, Obogashch. Rud 10 (4) (1965) 20-24. [7] P.B. Queneau, The kinetics of the dissolution of scheelite in basic solutions, Ph. Thesis, Univ. Minnesota (1967). [8] A.N. Zelikman, G.A. Meerson, Metalluurgiya Redkikh Metallov, Metallurgiya, Moscow, 1973. [9] E.G. Avvakimov, Mechanical Methods of Chemical Processes Activation, Nauk, Novosibirsk, 1979, 250 PP. [10] N.N. Khavskii, A.A. Bershitskii, Use of ultrasonic oscillations for accelerating the autoclave soda leaching of scheelite, Sb. Mosk. Inst. Stali Splavov 53 (1969) 77-85. [11] A.A. Bershitskii, B.A. Agranat, C.C. Vil'chik, N.N. Khavskii, B.N. Shmaleii, Effects of ultra-sounds in the scheelite leaching under pressures, Sb. Mosk. Inst. Stali Splavov 60 (1960) 189-191. [12] A.A. Bershitskii, N.N. Khavskii, B.N. Shmaleii, I.A. Yakubovich, A.A. Kal'kov, I.A. Rumyantsev, Sodium leaching of scheelite in autoclave subject to the elastic vibrations, Sb. Mosk. Inst. Stali Splavov 60 (1970) 193-197. [13] N.N. Khavskii, A.A. Bershitskii, C.C. Vil'chik, B.N.A. Shmaleii, Power effect of ultra-sounds in leaching tungsten concentrates, Ul'travuka Met. Protsessakh 197 (1972) 62-66. [14] P.M. Perlov, Interaction of calcium tungstate with soda carbonate in the autoclave process, Obogashch. Rud. 3 (1) (1958) 25-34. [15] J. Nasu, Nippon Kogyokai-shi 78 (1965) 745.

J.P. Martins, F. Martins /14ydrometallurgy 46 (1997) 191-203

203

[16] P.B. Queneau, D.K. Huggins, L.W. Beckstead, Soda ash digestion of scheelite concentrates, in: Proc. Extraction Metallurgy and Refractory Metal Symp., 1981, pp, 237-267. [17] S. Yagi, D. Chin, Fluidized-solid reactors with continuous solid feed (II), Chem. Eng. Sci. 16 (1965) 372-379. [18] O. Levenspiel, Chemical Reaction Engineering, 2nd ed., Wiley, New York, 1972, Chap. 12. [19] W.E. Ranz, W.R. Marshall Jr., Chem. Eng. Prog. 48 (1952) 141-173. [20] P.N. Rowe, K.T. Claxton, Trans. Inst. Chem. Eng. 43 (1965) T231. [21] R.H. Perry, C.H. Chilton, Chemical Engineers' Handbook, 4th ed., McGraw-Hill International, 1987. [22] Y. Konishii, H. Katada, S. Asai, Kinetics of dissolution of scheetite in aqueous Na4EDTA solutions, Metall. Trans. B 18 (B) (1987) 331-337.