(20.1%) diopside, and 35- 55% (42.2%) calcite. The mean content of wollastonite and diopside (useful components of ceramics) is 50%. Five ore zones 100 ...
Glass and Ceramics
Vol. 52, Nos. 9 - 10, 1995
Science for Ceramics Production U D C 549.642.41.004.14
WOLLASTONITE RAW MATERIALS AND THEIR APPLICATIONS (A REVIEW) G. M. Azarov, 1 E. V. Maiorova, 2 M. A. Oborina, 2 and A. V. Belyakov 2 Translated from Steklo i Keramika, No. 9, pp. 13 - 16, September, 1995. Natural and artificial wollastonite is used in the production of ceramics, devitrified glass (sitall), and as a polymer filler. Wollastonite additives decrease the firing temperature of ceramics, improve their strength, decrease the firing and drying shrinkages and the temperature of formation of the vitreous phase. Due to the high dielectric parameters, wollastonite ceramics are used in electrical and radio engineering. The use of wollastonite in the ceramics industry has good prospects.
Wollastonite has good prospects as a raw material in the ceramics industry. In recent years its production has increased. The number of traditional suppliers of wollastonite in the world market (the USA, India, Finland, Yugoslavia, Mexico) has been increased by China and Greece. Wollastonite rocks have been discovered in the CIS too. The largest deposits are located in the Far East, Yakutia, and Siberia. Kazakhstan and Central Asia also possess wollastonite rocks. The economic situation in Russia is characterized by rising prices for raw materials and transportation costs, which has stimulated works on developing local sources of raw materials. High-quality wollastonite is shipped to Siberia from the Slyudyansk Deposit in the Irkutsk Region. The ore contains (here and below by weight) 25 - 3 8 . 2 % (29.9% on the average) wollastonite, 0.5 - 9.1% (3.6%) quartz, 8.6 - 32.5% (20.1%) diopside, and 3 5 - 55% (42.2%) calcite. The mean content of wollastonite and diopside (useful components of ceramics) is 50%. Five ore zones 100 - 390 m wide and 3 - 50 m deep are distingxlished in the deposit. The reserves ofwollastonite ores amount to 2.8 million tons in three separate (200 - 250 km apart) regions. Wollastonite from the Slyudyansk Deposit contains 52.5% SIO2, 0.20% A1203,0.11% Fe203,46.82% CaO, 0.13% Na20, and 0.36% H20. This composition characterizes a high-purity material. The content of iron oxide in it is much more advantageous than in ores from other known deposits. Sodium and aluminum impurities virtually do not affect the quality of most products; they only decrease the
melting temperature of wollastonite. Material of such quality is of high interest for the porcelain and faience industry and for production of other kinds of ceramics [1]. Iron and manganese impurities are very harmful. They decrease the melting temperature of wollastonite and worsen its dielectric properties. Such wollastonite cannot be used in the production of ceramics for electrical engineering. Wollastonite CaSiO 3 (the theoretical content of CaO is 48.25% and that of SiO 2 is 51.75%) is a white mineral with a chain structure. The ring radical S i 3 0 9 has a lattice constant along the b-axis equal to 7.3 x 10-s cm. The compound exists in two modifications, namely, oc-CaSiO 3 (pseudowollastonite) and 13-CaSiO3 (wollastonite proper). Pseudowollastonite crystallizes into a pseudohexagonal structure, whereas wollastonite has a triclinic crystal structure. At a temperature of 1125~ wollastonite is irreversibly transformed into pseudowollastonite with an insignificant change in the volume. The melting temperature is 1540~ the density of cxwollastonite is 3.09 x 103 kg/m 3, and that of pseudowollastonite is 2.87 x 103 kg/m 3. The Molls' scratch hardness is 4.5 - 6 . The temperature coefficient of linear expansion of czwollastonite is 11.8 x 10 -6 K -1, that of 13-wollastonite is 6.5 • 10- 6 K-1. Wollastonite crystals have an acicular structure, but filamentary and tablet crystals are sometimes encountered. When such crystals are broken, the fragments usually have an acicular shape, because the cleavage of wollastonite is more perfect along vertical faces than in the pinacoid direction [2]. In addition to natural wollastonite, the ceramics industry also uses its artificial variant. It is chemically purer and possesses a free crystal structure, which distinguishes it from
1 Irkutsk Engineering University, Irkutsk, Russia. 2 D. I. Mendeleev Russian Chemical Engineering University, Moscow, Russia.
237 0361-7610/95/0910-0237512.50 9 1996 Plenum Publishing Corporation
238
naatral wollastonite. The structural differences determine some differences in the physical properties (the sintering temperature of synthetic wollastonite is 100-200~ lower than that of natural wollastonite). The use of purer synthetic wollastonite is expedient at elevated temperatures (above 1100~ The presence of synthetic wollastonite in the mixture for production of tile substantially improves the strengh characteristics and the frost resistance, whereas mixtures with natural wollastonite have lower shrinkage. Many valuable properties of wollastonite are associated with the acicular structure of its crystals, and therefore synthetic wollastonite is used more rarely than natural wollastonite [3]. In the first turn, wollastonite is used as a raw material for various ceramics. Investigation of the dielectric properties of wollastonite ceramics has shown that it has high electrical resistivity. Wollastonite additives promote the formation of a dielectric phase having high electrical resistivity. This property can be used for creating high-frequency electronic equipment. Lead silicate and wollastonite have been used to prepare a material with ultrahigh electrical resistivity. This makes it applicable in electrical and radio engineering. Specialists in the USA have created electric insulating porcelain with 20% wollastonite. It is characterized by negligible leakage of electricity and can be used in electronic equipment and in the production of glow plugs for aircraft which are heated to 1000~ in operation [4]. The possibility of using wollastonite from the Leninabad Deposit in electroporcelain has been investigated. The specimens possessed sufficiently high parameters of mechanical and electric strength. This makes the mixture usable in the production of low-voltage electric insulators [5]. The calcite-wollastonite raw material from the Slyudyansk Deposit has been used to prepare electroceramics. Specimens of plastic mixtures were sintered at 11401190~ Two crystalline phases were determined, namely, wollastonite and mullite. The ceramics had good properties, i.e., electric strength of 32 - 35 kV/mm, dielectric constant of 5.5 -6.1, electrical resistivity of about 1014 ~ 9m, and mechanical bending strength of 160 - 180 MPa [6]. Ceramics with low dielectric loss have been prepared from a charge containing 5 5 - 71% wollastonite at a firing temperature of 1100- 1220~ [7]. The use of wollastonite in the production of electroceramics considerably reduces their cost. Some studies have been devoted to the creation of lowfiring wollastonite porcelain. The content of wollastonite in the mixture was 25%, the firing temperature was 1050 - 1200~ with holding for 3 h, which is several times less than in the case of classical porcelain. An overall decrease in the sintering temperature has been observed. All parameters of wollastonite porcelain are no worse than in conventional porcelain. After firing at 1200~ the strengh of wollastonite porcelain was 67 MPa, whereas the strengh of conventional porcelain after firing at 1250~ was 32 MPa. Power consumption in the case of wollastonite porcelain is lower [8]. It is a selfglazing material. In combination with the low firing temperature, this widens the possibilities of decorating such porcelain.
G . M . Azarov et al.
Glaze compositions have been developed based on wollastonite from the Slyudyansk Deposit. The glaze has good luster and spreadability, has fewer pricks, and its whiteness is better than required by the standard for first-grade chinaware. Wollastonite is used for producing glazes, glaze flit, fluxes, and color mordants. The content of flit in the glaze can be reduced to 20%, which decreases its cost [4]. Examples of compositions producing a lustrous white glaze with a 15% content of wollastonite can be found in the literature. Wollastonite increases the luster and improves the structure of glazes. A heat-resistant sheet material that can be used at 1000~ has been created based on filamentary wollastonite. The composition of the mixture includes 5 0 - 80% wollastonite (Application 54-48855, Japan). Wollastonite is often used in the production of building materials, in particular, facing tile. Added to tile mixtures, it produces tile with good physicomechanical properties, reduces their drying and firing shrinkages to a minimum, decreases wet expansion, increases the deformation resistance, and the heat and frost resistances. Depending on the nature and origin, wollastonite additives can amount to 15 -65%. When the articles are fired, new crystalline phases are formed in them, for example, anorthite CaO 9AI203 92SiO 2 . The density of anorthite (2.76 g/cm 3) is much lower than that of wollastonite (2.9 g/cm3). When anorthite is formed from woltastonite, the true density of the crystalline phases decreases and their volume increases. This impedes shrinkage of the mixtures [9]. As the ceramic mixture is heated to 950~ anorthite is formed intensely, and at 1050~ this process is accompanied by partial dissolution of quartz, cristobalite, and wollastonite. Further heating enriches the melt with anorthite and wollastonite, which increases the amount of the melt, decreases its viscosity, increases shrinkage, and worsens the deformation resistance [ 10]. The decrease in the viscosity of the melt intensifies sintering. Mullite formation in mixtures containing wollastonite shifts to lower temperatures and its range decreases (1200 1250~ [3]. The properties of such materials can be improved by changing the amount of anorthite formed. In addition to the appearance of less dense crystalline phases, shrinkage is decreased by the formation of a skeleton of differently directed wollastonite crystals in the ceramic mixture. At a ftring temperature of 1000- 1100~ wollastonite dissolves in the melt only partially and the dense skeleton is preserved, preventing a change in the initial volume [3]. As the articles are cooled after firing, the charge crystallizes and the needles of the wollastonite skeleton stick tightly to one another. As a result, a reinforced material is obtained, the contacts in the crystal structure become stronger, and the strength of the article is increased. An increase in the wollastonite content in the mixture considerably decreases the temperature of formation of the vitreous phase and increases its amount. With melting of the vitreous phase and the appearance of new crystalline phases,
Wollastonite Raw Materials and Their Applications
239
quartz and low TCLE of wollastonite ensure resistance to thermal impact in rapid firing cycles. The introduction of 65% coarse wollastonite into the charge has made it possible to fire tile at a rate of 2000~ [12]. Wollastonite additives in the tile charge decreased the firing temperature to 9 5 0 1050~ and ensured a high quality of articles after a rapid single firing. An increase in the wollastonite content intensifies sintering, because wollastonite is a strong flux at high temperatures (above 1100~ It stimulates the process of porcelain formation, decreases the firing temperature by 50 - 80~ and reduces the f'n'ing time [13]. In synthesis of slag sitalls wollastonite is sometimes the main crystalline phase emerging in crystallization of slag glass. Compositions involving wollastonite are used in the production of dielectrics [14]. A glass composition has been developed for production of a glass crystalline material (si-
the volume of the melt decreases and pores 1 5 0 - 200 gm in size emerge. Wollastonite changes the composition of the vitreous phase, which envelops the pore walls because of the change in the surface tension coefficient. The porosity of the ceramics increases [11]. The strength of wollastonite ceramics is relatively low, but due to the high porosity and the net structure it is virtually unbreakable. It is interesting that such ceramics withstand nail driving [1]. Some properties of the ceramics with different contents of wollastonite are given in Table 1. The acicular structure of the crystals promotes a strong depleting effect of wollastonite at low firing temperatures (within 1000- 1100~ In a ceramic mixture containing wollastonite, the vitreous phase is saturated with calcium ions and the number of new crystalline phases is high, which leads to a decrease in wet expansion. The low content of
TABLE1 Properties o f ceramics
Wollastonite
From natural uncon-
Content of wollasshrinkage,
water
tonite, %
%
15 - 2 0
7
8.6 - 9.4
20
2.26
0.074
wet
T C L E in the r a n g e o f
absorption, % expansion, % 2 0 - 6 0 0 ~ 10 - 6 K - 1
strength, M P a
frost resis-
firing temperature, ~
Application bending
tensile
compressive
tance, cycles
-
-
-
27 - 3 0
O v e r 50
-
55.5
-
91.2
-
Reference
Brick charge
[5]
1200 - 1 2 5 0 L o w - v o l t a g e
[4]
centrated w o l t a s tonite ore from the Bosaginsk Deposit From the L e n i n a b a d Deposit
porcelain
Synthetic
15-25
From the B o s a g i n s k
20 - 30
6,4 - 6
0.5
17.19 - 17.6
21.7-22,4
30
0.6
16 - 17
-
14-15
-
-
-
1020
F a i e n c e f a c i n g tile
[15]
6.87 - 7.12
23 - 25
-
3 5 . 5 - 37
5 0 - 100
1050
Building ceramics
[16]
-
26
-
-
-
1000 - 1 0 9 0 P l a t e s for interior
[17]
Deposit From the K h a i r u z o v
0.012 - 0.014
Deposit
facing of a p l e a s a n t light-pink color
Synthetic Bosaginsk wollas-
40
0.8 - 0.9
t 4 . 5 - 15.5
-
17 - I 8
-
30 - 45
6 . 2 9 - 4.4
16.33 - 17.4
0.05 - 0 . 0 4
6.6 - 8.3
2 3 . 6 - 15.5
-
3 0 - 50
Lower
0
-
4 8 - 63
-
-
-
32.4 - 48.5 50- I00
1050
F a c i n g tile
[18]
1050
Building ceramics
[16]
1015
Wollastonite
tonite concentrate OF 1 Natural, w i t h 4 8 . 9 % C a O and 5 0 . 9 % SiO 2
-
-
than conven-
[5]
porcelain with low firing t e m p e r a t u r e
tional
and g o o d translucence
Synthesized from
50
0.9 - 1.4
15.5 - 13.5
50
0 . 9 - 1.1
15.8 - 16.7
0.02
9 . 9 - 11.3
13 - 15
-
-
-
1070
F a c i n g tile
[19]
9.7
15
-
-
35
980
I n t e r i o r and exterior f a c i n g tile, w h i t e
[20]
phosphogypsum Synthesized from siliceous l i m e
crock with a pink tint Product o f processing phosphogypsum into
5 0 - 55
0 . 4 3 - 0.5
1 3 . 5 - 14.2
0.02
9 . 3 - 9.6
20.2-21
-
-
Facingtile
USSR Inventor's
sulfuric acid, con-
Certificate
tains 5 0 % pseudowollasonite
No. 1604791
Natural
65
0.5
6
22.3
-
-
1100 - 1 1 5 0 F a c i n g tile, r a p i d
[12]
firing Local Indian sorts
50 - 71
8 . 8 9 - 17.6
0.1 - 0 . 1 7
6.6 - 12.3
-
6 6 . 6 - 105
-
1100 - 1200 W o l l a s t o n i t e ceramics
[7]
240
gran) resembling granite with spherulitic inclusions. The main crystalline phase of sigran is [3-wollastonite. Ornamentation is obtained by forming bulges on the surface. Wollastonite crystals grow normal to the surface. Spherulitic inclusions improve the decorative properties of the material which can be used as a substitute for natural facing materials (marble and granite) for walls of buildings, columns, underground stations, and other structures. The material possesses the requisite physicomechanical properties (USSR Inventor's Certificate No. 907987). Glasses based on wollastonite, feldspar, and silicic acid melt at relatively low temperatures, which preserves the acicular crystals of wollastonite and produces an original ceramic. When the lime and sand in the glass charge are replaced by wollastonite, the power consumption in melting is reduced and the quality of the melt is improved. The color and structure of wollastonite make it usable as a strengthening filler for polymers, like talc and asbestos. It ensures higher stren~h, water and heat resistances, and low dielectric parameters compared to other fillers. Wollastonite is not harmful for human health. Its use in the paint and varnish industry gives high-quality white and bright-color varnishes. In the production of polyether, vinyl, and epoxy resins, wollastonite is used as a 50% filling pigment. Insulation of steel skeletons of buildings can be made of foam obtained by treating fmely ground wollastonite with an acid, which condenses in cooling. The foam forms due to the calcite impurity contained in wollastonite [4]. Wollastonite possesses a number of valuable properties which positively affect the quality of the products. Its use in the ceramic industry decreases drying and firing shrinkages, increases the strengh of the articles, decreases the sintering temperature and the temperature of vitreous phase formation, and gives ceramics with good dielectric properties and highquality glazes. Natural wollastonite is an inexpensive raw material and its use decreases the cost of the products. REFERENCES 1. B. Z. Chistyakov, Prospects for the Use of Wollastonite [in Russian], Nauka, Moscow (1982), pp. 15 - 18. 2. V. L. Balkevich, Engineering Ceramics [in Russian], Stroiizdat, Moscow (1984).
G . M . Azarov et al.
3. V. L. Balkevich and A. Yu. Kogos, "Sintering of ceramic mixtures with natural and synthesized wollastonite," Steklo Keram., No. I, 19-21 (1988). 4. V. V. Kozyrev, "Prospects of using wollastonite," in: Wollastonite [in Russian], Nauka, Moscow (1982), pp. 18 - 23. 5. F. Kh. Tadzhiev and R. A. Ismatova, "Use of wotlastonite in the composition of electroporcelain," Steklo Keram., No. 9, 22 - 23 (1987). 6. Yu. I. Alekseev, "Synthesis of wollastonite electrocerarnics on the basis of raw materials from Slyudyansk Deposit," in: Rational Use of Natural Resources in Siberia. Abstr. Rep. Sei. Conf. [in Russian] Tomsk (1989), p. 42. 7. S. K. Cakrovorty and P. Chattopadnyay, "Low-loss ceramics from wollastonite," Trans. Indian Ceram. Sot., 43(6), 165- 169 (1984). 8. T. K. Dan and K. Yayachandran, "Development of wollastonite-based porcelains for low firing temperature," Res. Ind., 31(3), 218- 225 (1986). 9. M. K. Gal'perina and N. P. Tarantul, "Phase changes in rapid firing of wollastonite-containing ceramic tile," Steklo Keram., No. 11, 20-21 (1985). 10. V. L. Balkevich, A. Yu. Kogos, and F. S. Peres, "Argillite-wollastonite mixtures in the production of tile," Steklo Keram., No. 8, 19-21 (1985). 11. V. Z. Abdrakhimov, "Phase composition of tile based on industrial waste," Steklo Keram., No. 12, 22 - 23 (1991). 12. Sainamthip Prinya and Reed Yames, "Fast-fed wall tile bodies containing wollastonite," Amer. Ceram. Soc. Bull., 66(12), 1726- 1731 (1987). 13. V. L. Kozlovskii and Yu. S. Krupkin, "Investigation of the properties of wollastonite-containing porcelain," in: Ceramic Raw Materials and Improvement of the Technology of Chinaware [in Russian], Moscow (1989), pp. 34 - 40. 14. N. M. Pavlushkin, Fundamentals of the Technology of Sitalls [in Russian], Stroiizdat, Moscow (I 973). 15. L. M. Saltievskaya, Z. A. Livson, and M. I. Ryshchenko, "Synthesis of wollastonite and its use in ceramic mixtures," Steklo Keram., No. 2, 2 2 - 24 (1974). 16. B. I. Nudel'man, Z. P. Nurullaev, M. M. Gol'dshmidt, and M. Sh. Usharova, "Use of Bosaginsk wollastonite for building ceramics," Steklo Keram., No. 9, 1 7 - 18 (1980). 17. M. K. Gal'perina and N. P. Tarantul, "Ceramic tile from raw materials of Kazakhstan," Steklo Keram., No. 12, 2 2 - 2 3 (1991). 18. M. K. Gal'perina, N. S. Lykhina, and N. P. Tarantul, "Facing tile based on synthetic wollastonite," Steklo Keram., No. 10, 16 - 17 (1980). 19. M. K. Gal'perina, N. P. Tarantul, and E. M. Khachumyan, "Use of wollastonite synthesized from phosphogypsum for ceramic tile," Steklo Keram., No. 8, 20 - 22 (1983). 20. M. I. Bariz, V. F. Pavlov, and I. Yu. Bushmina, "Tile mixtures containing wollastonite," Steklo Keram., No. 3, 19 - 21 (1984).
