Vol. 10 No. 4 Dec. 2003 Article 119: 1005-9784 (2003) 04-029?-04
J.
CENT.
SOUTH
UNIV.
TECHNOL
.
Decomposing scheelite and scheelite-wolframite mixed concentrate by caustic soda digestion S U N P e i - m e i ( ~ J ~ j O , LI Hong-gui(~j~t~ ~ ) , LI Yun-jiao(~J~_~3~), Z H A O Z h o n g - w e i ( ~ 6 p ~ ) , H U O G u a n g - s h e n g ( ~ l ~ E ) , S U N Z h a o - m i n g ( ~ ] ~ ] B~), L I U M a o - s h e n g ( ~ ] ~ ) (School of M e t a l l u r g y Science and Engineering, Central South U n i v e r s i t y , C h a n g s h a 410083, China) Abstraet:Scheelite and scheelite-wolframite mixed concentrates with different grades were decomposed by caustic soda digestion, and the effects of caustic soda(NaOH) dosage, the ratio of water to ore and the stirring rate on tungsten recovery were studied. The results show that under the conditions that the ratio of water to ore is 0.8, stirring rate is 500 r • min -1 and the heating time is 2.0 h at 160 *C, the recovery of tungsten reaches 98% for scheelite (w(WO~) is 65.04%) and wolframite containing high proportion of calcium (w(WO~) is 65.58%, w(Ca) is 7.53%), when caustic soda dosage is 2.2 and 2.0 times of theoretical value, respectively; the recovery of tungsten can be more than 98°/oo,98% and 9 6 ~ respectively for low-grade scheelite-wolframite mixed concentrate with 55.78~, 45.32~ and 25.21% WO3, when caustic soda dosage is 2.6,3.2 and 5.5 times of theoretical value, respectively. Key words: scheelite concentrate; wolframite with high proportion of calcium; low-grade scheelite-wolframite mixed concentrate; caustic soda digestion CLC number: 0645.12 ; O614. 61 + 3 Document code: A
1
INTRODUCTION
With the continual exploitation and developm e n t of t u n g s t e n materials, wolframite becomes less and less. Statistical data show that the proportion of wolframite is only 3 2 . 9 % and that of scheelite is 4 5 . 5 % in t u n g s t e n resources, the rest is scheelite-wolframite mixed concentrate and m o s t of newly explored tungsten is scheelite Ell. T h e r e f o r e , how to economically exploit scheelite becomes significantly i m p o r t a n t in developing t u n g s t e n industries in China. C u r r e n t l y , caustic leaching is applied to handle wolframite concentrate, and hydrochloric acid decomposition, soda digestion and mechanical activated caustic decomposition E2-41 are applied for scheelite concentrate. T h e hydrochloric acid decomposition has poor adaptability to the raw materials and only suits for high-grade scheelite concentrate. F o r scheelite concentrate containing high level of impurities a n d low-grade scheelite, there are difficulties such as low tungsten recovery and low product quality, etc. T h e soda digestion usually requires high t e m p e r a t u r e (above 200 °C) and high pressure (above 2 . 0 M P a ) , thereby high-quality equipment
and strictly controlling operation are demanded. T h e mechanical activation caustic decomposition is a newly developed technique for handling scheelite and scheelite-wolframite mixed concentrate in the late 1980s and has been successfully applied in the industries E~'63. T h e mechanical activation caustic decomposition has wide adaptability to the raw materials and can effectively handle scheelite and other different types of t u n g s t e n resource. H o w e v e r , the mechanical activation caustic decomposition is also short in the short life of equipment for the friction between steel ball and cylinder. In order to overcome this s h o r t a g e , caustic digestion is applied to handle scheelite and low-grade scheelite-wolframite mixed concentrate. 2 2.1
EXPERIMENT Raw materials T h e experimental raw materials are scheelite
concentrate, wolframite with a high proportion of calcium and low-grade scheelite-wolframite mixed concentrate. T h e i r chemical compositions are s h o w n in T a b l e 1. N a O H is of industrial purity.
Received date: 2003-01-16; Accepted date: 2003-06-01 Correspondence: SUN Pei-mei, Professor; Tel: +86-731-8830476; E-mail: hgli@mail, esu. edu. en
Journal CSUT Vol. 10
298
Table 1
E x p e r i m e n t raw materials
w~ Ca
two materials is 2 . 2 and 2 . 0 ,
No. 4
2003
r~spectively, the
t u n g s t e n recovery reaches over 98% ; the suitable
No.
Raw material
W03
A
Scheelite concentrate
65.04
B
Wolframite with high proportion o{ calcium
65.58
7.53
decomposition, which can show the caustic soda
C
Scheelite-wolframite mixed concentrate I
55.78
6.23
concentration of slurry. T h e lower ratio of w a t e r
D
Scheelite-wolframite mixed concentrate II
45.32
9.80
E
Scheelite-wolframite mixed concentrate I[
25.21
4.00
ratio of w a t e r to ore is necessary for the process of
to ore,
the higher
caustic soda concentration,
which benefits decomposing.
2.2
Experimental equipment T h e experimental equipment is an autoclave with a cubage of 2 L and rotating speed of 0-1 000 r • min - ' . It is electric-heated and the allowing m a x i m u m pressure is 20 MPa. A n F K M - A exact controller was used to control rotating speed and t e m p e r a t u r e , while a t h e r m o c o u p l e and exact controller were used to measure t e m p e r a t u r e with a veracity of -q- 1 *C. T h e t u n g s t e n concentrate, N a O H and w a t e r were weighed and added into the reactor, then the reactor was sealed, the m i x t u r e was heated as well as stirred, subsequently filtrated and washed. It was necessary to control reverse reaction during the process by adding additive and other methods. Finally, the content of WOa in the residue was analyzed. T h e tungsten recovery was calculated based on mass fraction of insoluble WO3 in residue. T h e formula is given as follows: r/= [ 1 - - r Wr ( W 0 3 ) I X 100 O~ wc(WO~) where w , ( W O a ) is the mass fraction of WO3 in residue, wc(WO3) is the mass fraction of WO3 in concentrate, r is the ratio of residue to concent r a t e , ~ is tungsten recovery.
3 3. 1
RESULTS AND DISCUSSION
D e c o ~ i t i o n of scheelite concentrate and wolframite with high proportion of calcium U n d e r the conditions that t e m p e r a t u r e is 160 °C, heating time is 2 h, the effects of caustic soda d o s a g e ( t h e ratio of practical dosage to theoretical d o s a g e ) , the ratio of w a t e r to ore and the stirring rate on the tungsten recovery were studied. T h e results are s h o w n in T a b l e s 2-4 respectively. T a b l e s 2-4 show that for scheelite concentrate and wolframite with a high proportion of calcium, tungsten recovery increases with the increase of N a O H dosage with other conditions unchanged. T h u s , the suitable caustic dosage for the above
W h e n the ratio of
w a t e r to ore is lower, slurry viscosity becomes larger and its stirring becomes m o r e difficult, which does h a r m to reaction. T h e suitable ratio of w a t e r to ore is about 0.8. Because the lower ratio o{ water to ore can get higher caustic soda concentration, it is necessary to control a suitable stirring rate. It is s h o w n that the t u n g s t e n recovery increases with the increasing stirring rate f r o m 200 to 500 r • rain -1. T h u s , properly increasing stirring rate is helpful to reaction.
Table 2 Influence of N a O H dosage on tungsten recovery at ratio of w a t e r to ore of 0 . 8 and stirring rate of 500 r ° min -1 Sample No.
Type of raw material
dosage
NaOH
A-1 A-2 A-3 A-4 A-5 A-6 A-7 B-1 B-2 13-3 B-4 B-5 B-6 B-7 B-8
A A A A A A A B B B B B 13 B B
1.9 2. O 2. 1 2.2 2.2 2.2 2.3 1.6 1.6 1.8 1.8 1.8 2.0 2. 0 2.0
wr(woa)/~
7//~
17.69 7, 58 7.21 2.45 2.21 I. 50 1.35 11.50 9, 80 8.91 9.77 7.33 1.15 3.04 2.01
89.82 95.64 96.01 98. 66 98. 80 99. 18 99.26 93.51 94.47 94.97 94.49 95.86 99.37 98. 33 98.87
Table 3 Influence of ratio of w a t e r to ore (n) on tungsten recovery at stirring rate of 500 r • r a i n - ' Sample No. A-8 A-9 A-10 A-11 A-12 A-13 B-9 B-10 B-11 B-12 13-13 B-14
Type of raw material
NaOH dosage
n
A A A A A A B B B B B B
2.2 2.2 2.2 2.2 2.2 2.2 2.0 2.0 2.0 2.0 2.0 2.0
0, 6 0. 7 0. 8 0.9 1.0 1.1 0. 6 0. 7 0.8 0. 9 1.0 1.1
wr(WO3)/~
y/~
8.92 1.55 2.01 5.34 15.48 18. 63 4.25 1.15 2.56 4.53 12. 11 15. 42
84.93 99. 14 98.89 97.04 90.96 89.12 97.67 99.37 98.59 97, 51 92. 98 91.06
299
S U N Pei-mei, et al : Decomposing scheelite and scheelite-wolf r a m l t e m i x e d concentrate by caustic soda digestion
Table 4 Influence of stirring rate on tungsten recovery of scheelite concentrate at N a O H dosage of 2.2 and ratio of water to ore of 0.8 Type of raw Sample No. material
3.2
Stirring rate/ (r • rain -1)
Table 5 Results of decomposition of low-grade scheelite-wolframite mixed concentrate
w r ( W O s ) / ° ~ 7//~
Sample No.
Type oI raw material
NaOH dosage
wr(WOa)/%
,//%
Cq
C
2.2
8.17
93.11
A-14
A
200
12.71
92.57
(::-2
C
2.2
7.05
94.06
A-15
A
300
6.58
96.16
C-3
C
2.4
5.32
95.61
A-16
A
400
2.32
98. 72
C-4
C
2. 4
4.78
95.97
A-17
A
500
2.21
98.78
C-5
C
2.6
1.53
98.74
(:-6
C
2.6
1.27
98.95
C-7
C
2.6
1.27
98.95
Dq
D
2. 8
4. 38
94. 57
D-2
D
2.8
5.21
93.54
IY3
D
3.0
3.55
95.60
I~4
D
3.0
2.96
96.33
I~5
D
3.2
2.07
97.43
1~6
D
3.2
1.38
98. 29
13-7
D
3.2
1.61
98. 00
D~8
D
3.4
I. 40
98. 26
Decomposition of low-grade scheelite-wolframite mixed concentrate T h e effects of N a O H dosage on the tungsten
recovery were studied in dealing with different lowgrade
scheelite-wolframite
mixed
concentrates.
T h e results are shown in Table 5. Other conditions are as follows, the ratio of water to ore is 0 . 8 , t h e stirring rate is 500 r • min -1, temperature is 160 °C and the heating time is 2 h. Table 5 indicates that for different low-grade scheelite-wolframite mixed concentrates, such as C , D , E , when the caustic soda dosage is 2 . 6 , 3 . 2 and 5 . 5 , the tungsten recoveries can be more than 9 8 % , 98% and 9 6 % , respectively, indicating that the recoveries of tungsten all increase with increasing caustic soda dosage with other conditions unchanged for the tungsten raw materials. T h e effects of N a O H dosage on the tungsten recovery can be explained as follows: with a given ratio of water to ore, increasing N a O H dosage means the increase of caustic soda concentration in leaching slurry solution. T h e reaction is expressed as follows : CaWO4 q-2NaOH = Na2WO4 q-Ca(OH)z.
When reaction reaches equilibrium, equilibrium concentration quotient Kc can be calculated as follows • 4 w(Na2WO4) K ~ r±N~o~ Kc---- w2 ( N a O H ) r~:N~WO4 where w(Na2WO4) and w ( N a O H ) are mass fraction of Naz WO4 and N a O H at the condition of reaction equilibrium, respectively, r!na2wo' and r±NaOH are activity coefficient of NazWO4 and N a O H respectively. T h e experiments results show that the equilibrium concentration quotient Kc increases with the increase of temperature and N a O H concentration in the solution [7,s]. T h e r e f o r e , increasing N a O H dosage can lead to high tungsten recovery during the process.
I>9
D
3.4
I. I0
98. 64
13-10
D
3.6
0.92
98. 86
D-11
D
3.6
1.32
98.36
E-1
E
4. 0
3.78
88. 45 87.90
E-2
E
4. 0
3.96
E-3
E
5. 0
I. 36
95. 85
E-4
E
5.0
1.93
94. 11
E-5
E
5.5
1.09
96.67
E-6
E
5.5
1.08
96. 70
E-7
E
5.5
0.90
97.25
For a given tungsten recovery, N a O H dosage for different types of raw material depends on the mass fraction of WO3 and Ca in the material. Normally, the lower the WO3 grade and the higher the calcium content in the material, the greater the N a O H dosage. It also shows that the caustic dosage for scheelite concentrate and wolffamite concentrate with a higher content of calcium is greater than that for general wolframite concentrate. Similarly, for the three different low-grade scheelitewolframite mixed concentrates such as C , D , E , the results are the same as above. With the condition of N a O H dosage unchanged, in order to ensure higher N a O H concentrate in the process of decomposition, the lower ratio of water to ore is required. And in order to ensure mass transferring frequently in the process of reaction, the stirring rate should be controlled suitably. 4
CONCLUSIONS 1) Using caustic soda digestion to decompose
300
dournol CSUT
tungsten concentrates, tungsten recovery reaches over 98% for decomposing scheelite concentrate with 6 5 . 0 4 % W O s , when N a O H dosage is 2 . 2 , the ratio of water to ore is 0.8 and the stirring rate is 500 r * min -a at 160 *C for 2.0 h. 2)
For decomposing wolframite concentrate
with a high proportion of calcium ( 6 5 . 5 8 % WO3, 7.53%Ca),
the tungsten recovery reaches over
98 % , when caustic soda dosage is 2.0 times of the theoretical value. 3) For decomposing low-grade scheelite-wolframite mixed concentrates with 5 5 . 7 8 % , 4 5 . 3 2 % and 2 5 . 2 1 % WO3, the tungsten recovery reaches over 9 8 ~ ,
98~
and 96°~ respectively when the
caustic soda dosage is 2 . 6 , 3 . 2 and 5 . 5 times of theoretical value.
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