PRODUCTION
FIRING
CAUSTIC
MAGNESITE
DUST
N. F . B u g a e v , K. V Simonov, A . G. B e l o g r u d o v , A . I. K o r z h e n e v s k i i , E. P. Mezentsev, V. A P e r e p e l i t s y n , a n d N. N. B a r a n o v a
UDC 666.85:536.421. 5
C a u s t i c m a g n e s i t e dust r e m o v e d f r o m the flue g a s e s of r o t a r y and shaft f u r n a c e s , h e a t e d with oil and n a t u r a l g a s , c o n t a i n s a lot of m a g n e s i a (91-93%, b a s e d on the c a l c i n e d weight) and is a v a l u a b l e pro-duct f o r the p r o d u c t i o n of r e f r a c t o r i e s . However, as a r e s u l t of the p o o r s i n t e r a b i l i t y of the c a u s t i c dust, f i r i n g it p r o d u c e s m a g n e s i t e p o w d e r s of high p o r o s i t y (up to 40%) which h i n d e r s the p r o d u c t i o n of p e r i e l a s e - s p i n e l , m a g n e s i t e - c h r o m i t e r o o f and o t h e r p r o d u c t s f r o m t h e s e m a t e r i a l s , S e v e r a l p a p e r s [1-3] have d e a l t with the s i n t e r i n g of c a u s t i c dust and the u s e of the r e s u l t i n g p o w d e r s . T h i s p a p e r d e a l s with the r e s u l t s of w o r k a i m e d at obtaining d e n s e p o w d e r s by f i r i n g c a u s t i c dust in r o t a r y f u r n a c e s , 90 m long and 3.5 m in d i a m e t e r , using the d r y m e t h o d with s i n t e r i n g additives 9 The dust a p p r o a c h i n g the r o t a r y f u r n a c e f o r f i r i n g c o n s i s t s of a m i x t u r e of v a r i e t i e s of dust t a k e n f r o m d i f f e r e n t f u r n a c e s and having d i f f e r e n t p h y s i c o c h e m i c a l p r o p e r t i e s (Table 1) and a l s o d i f f e r e n t s i z e g r a d i n g s and p h a s e c o m p o s i t i o n s (Table 2). The dust c o n s i s t s ( F i g . l ) of p a r t i c l e s of r a w (MgCO3, N o = 1700), the c a u s t i c ( c o m p l e t e l y d e c a r bonated, N = 1 . 6 0 4 - 1 718) and d e a d burnt m a g n e s i t e ( p e r i c l a s e , N a p p r o x i m a t e l y 1.737). F u r t h e r m o r e , t h e r e a r e s i l i c a t e s and a l s o p a r t i c l e s of c a r b o n a c e o u s s u b s t a n c e and b r u c i t e (big = 1. 585, Np = 1.570). The dust f r o m the r o t a r y f u r n a c e s in d e p a r t m e n t No.1 (TsMP-1) h e a t e d with oil, is c h a r a c t e r i z e d by a high d e g r e e of r e g e n e r a t i o n in the o r i g i n a l r a w m a t e r i a l s and a high quantity of c a u s t i c m a g n e s i t e . The dust r e m o v e d f r o m the r o t a r y f u r n a c e s in T s M P - 3 f o r f i r i n g c a u s t i c dust has a high content of d e a d b u r n t m a g n e s i t e p a r t i c l e s , c o m p o s e d of individual m o n o c r y s t a l s of p e r i c l a s e o r a g g r e g a t e a c c u m u l a t i o n s of its g r a i n s . The dust t a k e n f r o m the r o t a r y f u r n a c e s in the T s M P - 4 d e p a r t m e n t c o n t a i n s T A B L E 1 C h a r a c t e r i s t i c s of C a u s t i c Dust and F i r e d B r i q u e t t e s ~r Sampling site
Shaft furnace cyclones . . . . Shaft furnace cyclones*. . . . Cyclones of rotary furnaces, TsMP-1 . . . . . . . . . . . Cyclones of rotary furnaces TsMP-3 (firing raw magne site) . . . . . . . . . . . . . . Eleetrofilters of rotary furnac TsMP-3 . . . . . . . . . . . . Cyclones of rotary furnaces TsMP-3(firing caustic dust: Cyclones of rotary furnaces TsMP-4. . . . . . . . . . . . Electrofilters of rotary furnac TsMP-4 . . . . . . . . . . .
36.9
29.0
32.3 17.5 29.4 24.8 2.6
* Furnaces were heated with coke. M a g n e z i t F a c t o r y . T r a n s l a t e d f r o m Ogneupory, No.2, pp.5-14,
76
F e b r u a r y , 1967.
T A B L E 2. C o m p o s i t i o n
of C a u s t i c D u s t Grain-size
COITI
osition, %
Phase composition, % magnesite
C9
Sampling site
Shaft furnace cyclones . . . . . . . . . . Shaft furnace cyclones* . . . . . . . . . . . Cyclones of rotary furnaces TsMP-1 . i Cyclones of rotary furnaces TsMP-3 i (firing raw magnesite) . . . . . . . . I Electrofilters of rotary furnace TsMP-3 Cyclones of rotary furnaces TsMP-3 (firing caustic dugt) . . . . . . . . . . Cyclones of rotary furnaces TsMP-4.. Electrofilters of rotary furnace TsMP-4
9
L
A
~
c-l, o
3.0 0.4 0.6
36.6 4.6 3.8
25.4 16.2 15.7
17.2 20.0 15.4
7.8 i8.8 ;4.5
35--40 25--30
0.2
6.8
19.0 3.0
16.8 5.0
3.8 3.8
%2 12.1 0.I
0.4 1.2 0.I
4.0
o
o
o
v
10-15
35-40 60-65
I 20-25 15--20
!--3 1--3
;7.2 ~I.6
45-50 40-45
20-25 30-35
15-20 10-15
1--2 1--2
~7.8 ~0.0 19.9
8-10
40-45
40-45
1--3
30-35
45--50
7--10
1--3
15'2o , 5 1--1.5 <1
<1
<1
~i
<1
I
* Furnaces were heated with coke.
a predominance burnt magnesite
of caustic magnesite- mainly of large fractions. There are smaller quantities of dead in the form of monocrystals, subjected to substantial recrystallization.
The caustic dust from the rotary furnaces heated with natural gas, and from the rotary furnaces heated with oil, differ mainly by the contents of sulfur compounds: in the remaining dust they are about the same. The content of sulfur inthedust (calculated as SO4) when heating with natural gas does not exceed i%.. To clarify the possibility of sintering, each dust was pressed at a pressure of 400 kg/cm 2 into briquettes, which were fired in an industrial tunnel kiln at 1700~ It was found that the dust, consisting mainly of coarsely dispersed particles and containing little silica and sesquioxides sintered especially badly. With increase in the content of the particles of raw and caustic magnesite, with a corresponding reduction in the quantities of dead burnt magnesite, sintering improved markedly. Dust from the different furnaces was fed into the rotary furnace for firing in different proportions, which led to substantial variations in the density of the finished magnesite powder. Six experimental batches of magnesite pmvder were prepared by firing a mixture of caustic dust in the rotary furnace, using chromite and titanomagnetite concentrate as sintering additives. The sintering additives obtained from the crushing-enrichment plant No.2 (DOF-2) were fed along an overhead cable way into a bunker into the tube mills (where it is necessary to grind the additives) and then into the bunker over the rotary furnaces or directly into the bunker over the furnaces, if the additives were not ground. The additives were batched into the mill by plate feeders, and the caustic dust by box feeders. The finely milled material was fed pneumatically from the mill into the bunker ever the furnaces.
Fig. I. Caustic dust extracted by cyclones of furnaces No.7 and 8 T s M P - 3 : 1) r a w m a g n e s i t e ; 2) c a u s t i c m a g n e s i t e ; 3) d e a d b u r n t m a g n e s i t e , Transmitted light, x200. Without a n a l y z e r ,
The titanomagnetite concentrate and the finely milled chromite with a moisture content of more than 1% tended to cake and stick in the bunkers, which hindered accurate batching. B a t c h i n g of t h e a d d i t i v e s w a s c o n t r o l l e d t h r o u g h t h e c o n t e n t of C r 2 0 a a n d T i C 2 in t h e f i n e l y m i l l e d m i x t u r e a n d in t h e p r e p a r e d powder. T h e a m o u n t of a d d i t i v e s s e n t f o r g r i n d i n g o r f i r i n g w a s checkedperiodically b y w e i g h i n g s a m p l e s t a k e n in u n i t t i m e f o r c h r o m i t e and t i t a n o m a g n e t i t e c o n c e n t r a t e .
77
TABLE 3. C h a r a c t e r i s t i c s of Caustic Dust f o r P r e p a r i n g E x p e r i m e n t a l Batches Content of oxides (calculated on calcined weight),% No. of batch 1 2 3 4 5 6
_
{
_
_
s 2.7 2.5 2.3 2.4 2.3 2.1
T A B L E 4. C h a r a c t e r i s t i c s
~o .~
c.o Not det. "3.3" 3 1 30
Loss on ignition %
SO
2.5 [ 93.0 2.2 I 92.5 2.3 91.5 Not deter mined ] 2,5 ] 92,2 3.0 91.7
1,4 1,7 2.1
]Content of IParticles [finer than ]0.09mm,%
17,8 19.4 10.1 6.0 24,3 7.7
2.6 2.6
-72.7 85.2 -80,4
of A d d i t i v e s
Material
o. o~
[ I
8iO~.
RzO3
1 2 3 4
[Ungroundchromite(fraction0-1mm). [ 8.2 ]Finely milled chromite . . . . . . . ,..[ 6.5 ]Same . . . . . . . . . . . . . . . . . . . . . . ]6.6 [Chromite for preparing finely milled I ] mixtures(fraction0-lmm) . . . . . . [ 5.0 5 [ Titanomagnetite concentrate (unground, [ [ fraction 0-0.5 mm) . . . . . . . . . . . [ 1.9 6 [ Titanomagnetite concentrate for prepar-[ [ ing finely milled mixture . . . . . . . [ 2.0
CrzO 3
Content of [ "~ ~ grains finer CaO[ MgO[ ~ ~'[than0.06
TiO~
20 ] 51.3 [ -- ] 0.8 [ 18.6] 2.7 [ ot det. ] 48.2 [ - [ 8.0 ] 19.7 [ 3.8 [ 22.6 1 4 6 . 8 1 - - [ 0 . 2 t 1 9 . 6 I 4.5 t [ 0.1 / 1 8 . 8 /
3.1 I
13.2 87.1 80.8
22.4
I 50.4 / --
54.6
[
--
]42.5[ 0.1 [ 3.6[ 3.6 ,
26
50.7
/[
-
/ [/ [46.0
26
0 [/ 2.4 [t 2.9 [/
--
T A B L E 5. C h a r a c t e r i s t i c s of F i n e l y M i l l e d M i x t u r e s of C a u s t i c D u s t with C h r o m i t e and T i t a n o m a g n e t i t e C o n c e n t r a t e Content, %
No.of batch SiO2 [ AIsO~
4 ]30t.7 NOtl?t ,
6
CGOs t Fe20~ CaO
MgO
TiO2
Content of ,IOS8on grains SOs igni- finer than lUon / 0.09, %
I
10,2
Notdet. 3.2
2.0 [ 66.0 2,4 1 80,5
G
2,0 / 5.8 [ 2.2 8.0
96.0 94,6
T h e c h e m i c a l c o m p o s i t i o n and t h e f i n e n e s s of t h e c a u s t i c d u s t and a d d i t i v e s u s e d f o r m a k i n g t h e e x p e r i m e n t a l b a t c h e s a r e g i v e n in T a b l e s 3 and 4. T h e e x p e r i m e n t a l b a t c h e s of dust w e r e r e l a t i v e l y u n i f o r m in c h e m i c a l d i f f e r e n t in r e g a r d to l o s s u p o n c a l c i n a t i o n .
composition
and w e r e v e r y
In p r e p a r i n g t h e f i r s t and f o u r t h e x p e r i m e n t a l b a t c h e s w e f i s e d c h r o m i t e r i c h e r in C r 2 0 3 t h a n f o r t h e s e c o n d and t h i r d t e s t b a t c h e s . T h e c h r o m i t e u s e d f o r t h e p r o d u c t i o n of t h e s e c o n d b a t c h w a s m u c h finer. The finely ground mixture with the addition of chromite contained almost double the amount of silica compared with the finely ground mixture containing the additive of titanomagnetite concentrate (Table 5). shown
The composition in Table 6.
of the batch and the weight of the experimental
batches of magnesite
powders
are
During the calcination of the finely milled mixtures of caustic dust containing the chromite (batch No.4) and the titanomagnetite concentrate (batch No.6) the furnace was operated on a highly forced heating cycle (Table 7). The temperature cycle in the dust chamber and control of the fuel input ensured
78
T A B L E 6. C o m p o s i t i o n of B a t c h e s a n d W e i g h t of P o w ders m
Composition of batch
.x=
Component
S
Caustic dust Unground chromite
91 --94 6-9
Caustic dust Finely ground chromite
88-92 8-12
Caustic dust Finely ground chromite
88- 92 8--12
Caustic dust Chromite (combined grinding)
80 20
Caustic dust Titanomagnetite (unground)
95-96
Caustic dust Titanomagnetite concentrate (combined grinding)
96-97
4--5
m a x i m u m t e m p e r a t u r e d e v e l o p m e n t in t h e s i n t e r i n g z o n e of t h e r o t a r y f u r n a c e 9 W i t h t h e o r d i n a r y f i r i n g of c a u s t i c d u s t t h e f u r n a c e c y c l e is similar to the cycle used for obtaining experim e n t a l b a t c h No, 5. To obtain representative experimental b a t c h e s t h e c a l c i n e d p o w d e r s w e r e s t o r e d in i n d i v i d u a l c o n t a i n e r s and a n a l y z e d 3 - 1 0 h f r o m t h e m o m e n t of input into t h e f u r n a c e . T h e q u a l i t y of t h e p r e p a r e d m a g n e s i t e p o w d e r s w a s d e t e r m i n e d b y t h e i r g r a i n - s i z e and c h e m i c a l c o m p o sitions, the porosity and structural characteristics.
400
700
1630
T h e c h e m i c a l c o m p o s i t i o n of t h e g r o s s s a m p l e s of t h e e x p e r i m e n t a l p o w d e r s is shown in T a b l e 8, T h e p o w d e r s o b t a i n e d f r o m c a u s t i c d u s t w i t h a d d i t i o n s of c h r o m i t e , c o n t a i n e d a h i g h c o n t e n t of s i l i c a . T h e h i g h e s t c o n t e n t of MgO w a s s h o w n f o r p o w d e r s p r e p a r e d f r o m c a u s t i c d u s t w i t h a d d i t i o n s of t i t a n o m a g n e t i t e concentrate.
840 800
640 3--4
M a g n e s i t e p o w d e r with r e l a t i v e l y s m a l l c o n t e n t s of f r a c t i o n s f i n e r t h a n 0.5 r u m ( T a b l e 9) w e r e o b t a i n e d f r o m c a u s t i c d u s t c o n t a i n i n g an a d d i t i o n of 6-9% c h r o m i t e , f r a c t i o n 0 - 1 r a m , b u t t h e q u a n t i t y of o v e r t i r e d m a t e r i M in the p o w d e r w a s q u i t e high. T h e b u l k s a m p l e of t h e p o w d e r No.1 b a t c h c o n t a i n e d a l a r g e q u a n t i t y of s i l i c a (4.4%); t h i s a p p a r e n t l y i s e x p l a i n e d b y t h e r e l a t i v e l y high d e n s i t y of t h e f r a c t i o n 3 - 1 r a m , e q u a l on a v e r a g e to 15.9%. W i t h an i n c r e a s e in t h e g r a i n s i z e t h e p o r o s i t y n a t u r a l l y i n c r e a s e s : t h e p o r o s i t y of t h e g r a i n s m e a s u r i n g 1 0 - 8 m m w a s 17.7%, t h e g r a i n s m e a s u r i n g 20 m m - 23,5%. T A B L E 7. W o r k i n g C y c l e f o r R o t a r y F u r n a c e P r o d u c i n g E x p e r i m e n t a l B a t c h e s *
i
in gas pipes
530
Temperature the in dust material in chamber cooler
480~650
690 550--760
196 100--250
577 520--660
738 700--780
75 62--85
496 300--610
710 650--780
127 100--180
563
765
490--620
745--800
98 30--220
oil
Draft, mmindustWater"~ chamber in hot head
2.1
2.5
1.3
0.8--2.2
3,3 0.5--0,8
96 84---110
1.1 0.4--4,6
4,9 2,5--14,0
97 98--99
1,3 0,8--t .8
1,3 0,0--6.0
Composition of flue gases,% co= 25,4 22.4-- 28.0
o~
co
I. 6
O,05 0--0.4
0,6--3,4
14,0
13.2
m
i
512
698
460--560
650--750
148 83--198
595 560--660
774 740--800
117 40--220
100 83--i10'
3.9
2.1
"I .5--5,5
1,8--3,5
1,9
3.9
1.2--2,5
1.2--2,5
14.2 8,0--I5,6
1,0 0,4--3.6
itcr
13.4
14,2 12.7 10.2--15
0.7 0.4 -1,6
}ICT
12.0
* The numerator shows the average - the denominator, the ex~ema. 1" 1 mm water ~ 9.8 Nm2.
79
T A B L E 8 0 . C h e m i c a l C o m p o s i t i o n of E x p e r i m e n t a l M a g n e s i t e P o w d e r s (Bulk S a m p l e s ) , % No.of
extmri mental batches
SiO~
1 2 3 4 5 6
4.4 3.5 3.3 3.8 2.7 2.3
F%Os
203
A1203
~.5
D
5.7 5.2
2.1 3.6 1.3 1.3
m
3.5 5.1 4.6 3.8
).9
CaO
MgO
3.2 2.'/ 2.5 1.9 2.3 2.8
83.0 88.3 75.8 88.9 88.5
TiO 2
w
Loss on ignition
w
D
0.05 0.1 0.1 0.3 0.1
u
m
1.9
1.3
T A B L E 9. G r a i n S i z e C o m p o s i t i o n o f T e s t P o w d e r s , %* Fraction, mm
No. of bamh
>20
1 2a-10
_2o,_8__ 27.1 _ 8.8 11,8--9_;}.5122.0-88.{ 7--11,5
1 [
8.1 17.0--18.5
lo.ol 8,3 24.7 28.3 15.0--44,C 19.0.--85.{ 4.0--t7,, 3.3--11,0 7,6 6.4 . 32.5 30.9 16.6--5L~ 22 -48 ~ 3 . 0 - - 1 4 . 0 12.3 19.6 14,7 _ _ 17.3
i
12.5
8.L___ 6.8
2.4
_
5,v
]_2.~
~.6
11.0 --39.'.; 11,2--35.z
__8_.8
17,~__a
2,0-5,8 t8.0-1=.011.0-2 6 ,0-24.4 2.3
I
3,4
0,8
18.4
o,8-91~1o,6-5~ 1oC~--~,2 1 ~ 7.2 1 9 . o
I o.7
4.2 t, .4
13.0
{)--42.C 5.8--3,fl.( 3.o-18. 3,o-2o.o 2,0-1oT613-,o-14.8 [ ~ T 6 -
__??_.~ ..... ~L.7__. _ 8 . ~
15.2
0-~-~.~ 7.5-22.0
3.5--14,01~1--14,6 ] ~
:~,o-81.~ ~, 3-32.r
__ ~L._8__, lS,t_
5,6
AT--C0-~ l
17.o
OZ47~.O
3 .6 8,-8--4GT5
]2 .2
5.1--12. 7.0- 13.0 2 . 8 - 8 2~-'4T- ~6TJl1-s
* The numerator shows the average and the denominator the extrema.
W i t h t h e i n t r o d u c t i o n into t h e c a u s t i c d u s t of 8-12% f i n e l y m i l l e d c h r o m i t e t h e r e i s a s u b s t a n t i a l i n c r e a s e in t h e c o n t e n t of o v e r f i r e d m a t e r i a l in t h e p r e p a r e d p o w d e r ( g r a i n s c o a r s e r t h a n 20 ram) on a c c o u n t of a r e d u c t i o n in t h e a m o u n t of 4 - 1 m m f r a c t i o n . T h i s a p p a r e n t l y l e a d s t o i r r e g u l a r d i s t r i b u t i o n of t h e c h r o m i t e owing t o t h e f a c t t h a t t h e c o m p o n e n t s w e r e not f i r s t m i x e d . T h e c o n t e n t of t h e f r a c t i o n f i n e r t h a n 0, 5 m m i s p r a c t i c a l l y t h e s a m e a s in t h e f i r s t b a t c h of p o w d e r . W h e n t h e b u l k s a m p l e of p o w d e r w a s c r u s h e d t h e c o n t e n t of f r a c t i o n f i n e r t h a n 0.5 m m i n c r e a s e d t o 35%. The f i n e l y m i l l e d c h r o m i t e d i d not y i e l d a d e q u a t e l y d e n s e m a g n e s i t e p o w d e r . T h e p o r o s i t y of t h e s c r e e n e d f r a c t i o n 3 - 1 m m f o r t h e p o w d e r No.2 and 3 b a t c h e s w a s 23 and 25% r e s p e c t i v e l y ; t h e p o r o s i t y of t h e f r a c t i o n s 3 - 1 m m o b t a i n e d a f t e r c r u s h i n g t h e p o w d e r w a s 28% ( F i g 2, a and b). T h e r e w e r e no s i g n i f i c a n t d i f f e r e n c e s in t h e p o r o s i t y of t h e f r a c t i o n s o b t a i n e d f r o m d i f f e r e n t p o w d e r s . T h e r e w a s a t y p i c a l l y q u i t e i r r e g u l a r d i s t r i b u t i o n of o x i d e s in t h e v a r i o u s p o w d e r f r a c t i o n s : C r 2 0 3 w a s c o n c e n t r a t e d in t h e o v e r f i r e d m a t e r i a l s . A l a r g e i n c r e a s e in t h e c o n t e n t of SiO 2 at t h e e x p e n s e of a r e d u c t i o n in t h e MgO i s o b s e r v e d in t h e f r a c t i o n 1 - 0 . 5 r a m . F o r t h i s f r a c t i o n t h e r e i s a t y p i c a l i n c r e a s e in t h e c o n t e n t of c a l c i u m o x i d e . In t h e g r a i n s of p o w d e r m e a s u r i n g l e s s t h a n 0.5 m m t h e r e is a s h a r p i n c r e a s e in t h e a m o u n t of m a g n e s i a and a r e s p e c t i v e r e d u c t i o n in t h e c o n t e n t of SiO 2, CaO, and F e 2 0 3. V a r i a t i o n s in t h e c o n t e n t of s e s q u i o x i d e s in t h e i n d i v i d u a l f r a c t i o n s a r e s l i g h t . P e t r o g r a p h i c i n v e s t i g a t i o n e s t a b l i s h e d t h a t t h e g r a i n s of p o w d e r o b t a i n e d f r o m c a u s t i c d u s t with a d d i t i o n s of f i n e l y m i l l e d c h r o m i t e , c o n s i s t of 90-92% p e r i c l a s e with i n c l u s i o n s of s e c o n d a r y s p i n e l of c o m p l e x c o m p o s i t i o n , 6-8% s i l i c a t e s and 1% f r e e Ca(}, d e t e r m i n e d b y W h i t e ' s r e a c t i o n . T h e s i l i c a t e s c o n s i s t m a i n l y of m e r v i n i t e w i t h s m a l l c o n t e n t s of f i - 2 C a O . S i O 2 with n o r m a l o p t i c a l c o n s t a n t s . T h e f i n e l y m i l l e d c h r o m i t e i s u s e d up c o m p l e t e l y f o r t h e f o r m a t i o n in t h e p e r i c l a s e of s e c o n d a r y s p i n e l of
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Fig.2. Chemical composition and porosity of magnesite powders in fractions for the batches: a) second; b) third; c) fourth, d) fifth, e)'sixth; dashed lines show apparent porosity.
complex composition. Over the c r o s s section of the c o a r s e g r a i n s m e a s u r i n g 15-20 m m we o b s e r v e a zonal s t r u c t u r e . The internM p a r t of the g r a i n s is v e r y porous (the p o r e s constitute 50% of the volume). In the direction toward the p e r i p h e r a l sections of the grain, the p o r o s i t y falls sharply; the p o r o s i t y of the s u r f a c e skin is not m o r e than 10-15%. A s t r u c t u r a l f e a t u r e of the powder is the weak r e c r y s t a l l i z a tion of the p e r i c l a s e (Fig.3a). This is c o n f i r m e d by the fact that the p e r i c l a s e g r a i n s contain no internal pores. The size of the finer g r a i n s of p e r i c l a s e v a r i e s in the range 0.008-0.05 m m (mainly 0.01-0.02 ram). The distribution of the silicates over the c r o s s section of the g r a i n s is r e l a t i v e l y uniform. We note a g g r e g a t e s of fine g r a m s of p e r i c l a s e . After firing the finely milled mixture of caustic dust with c h r o m i t e , t h e r e is a substantial reduction in the quantity of o v e r f i r e d m a t e r i a l and a r e s p e c t i v e i n c r e a s e in the content of the 8-4 m m fractions, and also of fractions finer than 0.5 mm. The g r a i n s of powder are brittle. T h e r e f o r e , during crushing we obtain up to 50% g r a i n s finer than 0.5 mm. Combined grinding of a mixture of 80% caustic dust and 20% c h r o m i t e yields a powder of f r a c t i o n 3-1 m m with an apparent p o r o s i t y of 11.7%; the p o r o s i t y of the powder, f r a c t i o n 3-1 ram, obtained by c r u s h i n g was 16%. The powder was obtained by i n c r e a s i n g the heat load in the furnace. The p o r o s i t y of the c o a r s e g r a i n s of powder are v e r y different f r o m the p o r o s i t y of the 3-1 m m g r a i n s (Fig.2c). By firing finely milled m i x t u r e s of caustic dust containing c h r o m i t e we obtain powder, the o v e r f i r e d content of which contains the least quantity of Cr203. The content of MgO in the o v e r t i r e d m a t e r i a l is quite high. The m i n e r a l composition of the powder is mainly p e r i c l a s e (about 90%), the r e f r a c t i v e index of which is i n c r e a s e d (N approximately 1.780), apparently, owing to the p r e s e n c e of m a g n e s i o f e r r i t e in a solid solution of p e r i c l a s e . The silicates consist of f o r s t e r i t e and montichellite in amounts of 7-10% by volume. The s t r u c t u r e of this powder is quite uniform. No a g g r e g a t e s of fine g r a i n s of p e r i c l a s e were
81
Fig.3. M i c r o s t r u c t u r e of magnesite powders: a) powder obtained with addition of finely m i l l e d c h r o m i t e [I) internal p a r t of the grain; II) surface skin]; b) powder obtained by firing finely milled m i x t u r e of caustic dust and c h r o m i t e ; c) powder obtained by firing finely milled m i x t u r e of caustic dust with titanomagnetite concentrate. l~eflected light. • 90.
o b s e r v e d . The silicates are distributed quite u n i f o r m l y o v e r the c r o s s section of the grain. The size of the c r y s t a l s of p e r i c l a s e is 0.04-0.07 m m (Fig.3b). As a r e s u l t of the high content of the silicate phase and rounded periclase grains,, we .observe weak development of a direct bond between the grains of p e r i c l a s e with each other. The absence of p o r e s inZ.087 Z.09~ side the c r y s t a l s of p e r i c l a s e suggests r e l a t i v e l y slow Z4! growth. In the powder f i r e d with additions of titanomagnetite c o n c e n t r a t e we o b s e r v e a uniform d i s t r i b u Z,4~ tion of the oxides TiO2, A12OI, and Fe201 (Fig.2dande). 9
2.68
a
~.
b
Fig.4. Sections of x - r a y lines f o r o r d i n a r y magnesite powder without additions (a}, and powder with additions of titanomagnetite conc e n t r a t e (b}, combined grinding: (A) CaO. MgO'SiO2; ([:]) MgO; (o) MgO. Fe203; (• 2MgO. TiO 2.
82
During the firing of ground and unground caustic dust containing 3-5% titanomagnetite concentrate we obtain adequately dense magnesite powders with a p o r o s i t y of 12.6-12.9%. The quantity of grains finer than 0.5 m m in the powder of finely milled dust is r e l a t i v e l y high and amounts to 23.2%. The powders are brittle. During crushing the content of the f r a c t i o n f i n e r than 0.5 m m i n c r e a s e s to 50%. This hinders the use of the powder f o r the production of c o m m e r c i a l products. The grains of powder consist mainly of p e r i c l a s e (90-95%). According to calculations based on chemical
analysis, the periclase contains about 17% magnesioferrite, The silicates (5-7%) in the powder consist mainly of montichellite. The central part of the grain of the powder has a higher porosity than the peripheral 3ections. The structure of the grains is uniformly finely crystalline; ~he periclase crystals are 0.03-0.05 ram. The silicates are uniformly distributed and form fine films around the crystals. The grains conrain large quantities of the direct bond MgO-MgO. The part of the grain of periclase contains internally fine (about 0, 03 ram) closed pores, formed as a result of the trapping of the boundary pores during rapid grain growth (Fig.3c). MgO,
The closed porosity in the grains suggest the relatively high rate of selective recrystallisation of due to the mineralizing action of the addition of titanomagnetite.
The presence in the experimental powder of the compound analysis (Fig.4 a and b).
2MgO'TiO 2 was confirmed by x-ray
CONCLUSIONS Six experimental batches of magnesite powder were obtained for preparing magnesia products by firing caustic dust in a rotary furnace. To improve the sintering, chromite or titanomagnetite was added to the dust. The final conclusions on the optimum cycle for the production of dense magnesite powders from caustic dust will be made after we have obtained data on the stability in service of products made frorn these powders. LITERATURE 1. 2. 3
CITED
V . A . Bron et al., Ogneupory, No.5, p.221 (1964). V . A . Bron et M., Ogneupory, No.2, p.ll (1966) V . A . Bron e t a / . , Trans. VostIO, Izd-vo. ~Metallurgiya ~, No.5,
pp.196-204.
A l l a b b r e v i a t i o n s of p e r i o d i c a l s in the a b o v e b i b l i o g r a p h y are l e t t e r - b y - l e t t e r t r a n s l i t e r a t i o n s of the a b b r e v i a t i o n s as g i v e n i n the o r i g i n a l R u s s i a n j o u r n a l . S o m e or a l l o f t h i s p e r i o d i c a l l i t e r a t u r e m a y ,~vell b e a v a i l a b l e in E n g l i s h t r a n s l a t i o n . A c o m p l e t e i i s t of the c o v e r - t o c o v e r E n g l i s h t r a n s l a t i o n s a p p e a r s at the back of the f i r s t i s s u e of t h i s year.
83
