preparation of porous ceramic materials based on wollastonite using ...

Refractories and Industrial Ceramics

Vol. 52, No. 3, September, 2011

PREPARATION OF POROUS CERAMIC MATERIALS BASED ON WOLLASTONITE USING SILICON-CONTAINING COMPONENTS A. T. Volochko,1 K. B. Podbolotov,2 and A. A. Zhukova1 Translated from Novye Ogneupory, No. 5, pp. 24 – 29, May 2011.

Original article submitted January 14, 2011. Results are provided for evaluation of the possibility of preparing porous ceramic materials using synthetic wollastonite and silicon-containing components by chemical pore formation. The mechanism of pore formation, and also chemical processes during material drying and firing are studied. The physicochemical properties and microstructure of the material obtained are studied. Keywords: porous ceramic material, wollastonite, silicon-containing components, chemical pore formation method.

are prepared by pouring previously prepared liquid-binder slip, of the required composition and consistency, into special dismountable molds in order to obtain finished objects of different configuration, i.e., bricks, blocks, bars, and plates. The porous objects have an apparent density of 200 – 600 kg/m 3, thermal conductivity of 0.12 – 0.35 W/(m·K), with an ultimate strength in compression of 1.0 – 4.0 MPa. However, for occurrence of SHS it is necessary to use a reducing agent in the form of metal (aluminum and its alloys), which is not always justifiable. Studies are known devoted to preparing wollastonite lightweight objects, which have found extensive use in the aluminum industry as a lining for troughs, in the manufacture of casting equipment during pouring of aluminum, and also for heat insulation of eletcrolyzers, melting furnaces and mixers of the aluminum industry [1 – 5]. In the A. S. Berezhnyi UkrNIIO technology has been developed for wollastonite lightweight objects with an apparent density of 1.0 g/cm3 and with an ultimate strength in compression of 3.6 MPa, prepared by solid-phase synthesis from calcium and silicon-containing materials [6 – 8]. Use of wollastonite makes it possible to prepare porous objects with quite high strength due to its structural features; the application temperature for these objects is not above 1200°C. This work is devoted to preparing new heat insulation materials based on wollastonite by chemical pore formation using silicon-containing components.

INTRODUCTION Wollastonite is a natural calcium silicate with the chemical formula CaSiO3. It is white in color with gray or brown tints, distinguished by chemical purity, and it contains an insignificant amount of harmful impurities in the form of manganese, iron and titanium oxides. For wollastonite there is a typical acicular crystal structure, which determines its main use as a micro-reinforcing filler. In view of the worsening ecological situation it should be noted that wollastonite is a replacement for such carcinogenic substances as asbestos, and talc. Until recently asbestos heat insulation materials have been used within Belorussia in all metallurgical enterprises. During their prolonged operation there is breakdown, as a result of which asbestos dust appears in production areas, which is a source of oncological pulmonary diseases for humans. In this connection, replacement of asbestos by more ecological heat insulation materials is an important task. In order to resolve the problem different technologies are used for creating porous materials for engineering purposes using the method of self-propagating high-temperature synthesis (SHS), gas formation in the hardening stage followed by heat treatment, and introduction of combustible additions during synthesis, etc. Recently new refractory, especially light, SHS concretes have been developed, prepared by cold expansion, followed by heat treatment and SHS. The objects 1 2

GNU Physicomechanical Institute of NAN Belorus’, Minsk, Belorus’ Republic. Belorussian State Technological University, Minsk, Belorus’ Republic.

186 1083-4877/11/05203-0186 © 2011 Springer Science+Business Media, Inc.

Preparation of Porous Ceramic Materials Based on Wollastonite WORK METHODOLOGY Synthetic wollastonite grade Casiflux 75 was used for this study. The silicon-containing components were commercial grade silicon (GOST 2169), ferrosilicon grades FS45 and FS75 (GOST 1415), and the binder and active component used for preparing objects was a water glass solution (GOST 13078). A charge was prepared by mixing wollastonite and silicon-containing component in dry form in order to prepare a uniform mix, which was mixed with water glass solution. Test specimens were prepared by ramming a moist mix into dismountable metal or plastic molds. For completion of expansion and hardening of specimens they were dried at 100 – 120°C to complete moisture removal, and then heat treated in an electric furnace at 800 – 1100°C for 1 h. Then specimen physicochemical properties were measured by standard procedures, and their phase compositions and structure were determined. X-ray phase analysis (XPA) was performed in a DRON-3 instrument, and diffraction patterns were interpreted using a ICDD PDF-2 card index. Material structure was studied by means of an optical microscope. RESULTS AND DISCUSSION Numerous methods exist for giving materials a porous structure. By summarizing and combining methods, having common features, they may be reduced to the following: – introduction into a starting mix of porous filler, i.e., natural or artificial; – expansion in the course of heat treatment of a whole mix or its individual components; – introduction into a mix and subsequent removal (evaporation, distillation, dissolution, firing) of additions, leaving pores; – phase formation during firing, connected with an increase in porosity; – putting air into a suspension or melt and securing the bubbles formed; – formation in a suspension or melt of gas bubbles as a result of chemical reaction or decomposition of additions [9]. The last method was used in this work, which in scientific publications is encountered as a method of chemical pore formation. The basis of this method is introduction of gas with chemical reaction of components, for example with reaction of metal (in our case silicon, ferrosilicon) with bases. Here one of the components of the system may be water glass, during whose hydrolysis in aqueous solution there is formation of a sufficient amount of sodium hydroxide and a strongly alkaline medium is created. Hydrogen is liberated during occurrence of chemical reaction of silicon with alkaline solution, which promotes pore formation. Reaction of components may be represented by the following chemical processes:

187 Na2O·SiO2 + 2H2O = 2NaOH + SiO2 + H2O,

(1)

2NaOH + Si + H2O = Na2SiO3 + 2H2.

(2)

Today in the market 1 kg of silicon costs 9800 bel. rub., and ferrosilicon costs 6200 bel. rub, and therefore use of ferrosilicon is preferable from an economic point of view. Research was performed for use of a wollastonite base, to which silicon and ferrosilicon FeSi grades FS45 and FS75 were added in an amount from 2.5 to 10%. Specimens were prepared according to the operating methodology described previously. It was established that in order to create high porosity it is necessary to use dilute water glass solutions with a density of 1100 – 1150 kg/m3. Dilution of water glass with water assumes presence of more favorable conditions for occurrence of hydrolysis in accordance with reaction (1); correspondingly there is less time for reaction of silicon with alkali by reaction (2). However, water glass concentration below a certain limit makes it impossible to provide the required final object strength. With use of ferrosilicon the reaction occurs similarly, with the exception of formation of iron compounds: 2NaOH + FeSi + H2O = Na2SiO3 + 2H2 + Fe.

(3)

On reaction with water iron in an alkaline medium and in the presence of air oxygen forms iron (III) hydroxide: 2Fe + 3H2O + 3/2O2 = 2Fe(OH)3.

(4)

Reactions (1) and (3) are exothermic, they occur with liberation of heat, which promotes acceleration of both hydrolysis and reaction in mixes, and also provides less expenditure for heat during drying an object in view of removal of a significant amount of water as a result of evaporation with self heating of the system. During drying of specimens there is polymerization and hardening of water glass, as a result of which there is assembly of specimen strength: Na2O·nSiO2·mH2O = Na2SiO3 + (n–1)SiO2 + mH2O. (5) However, during firing the following processes occur preferentially: decomposition of iron (III) hydroxide: 2Fe(OH)3 = Fe2O3 + 3H2O,

(6)

oxidation of unreacetd iron with oxide formation: 4Fe + 3O2 = 2Fe2O3,

(7)

formation of ferrite with fusion of iron (III) oxide with sodium hydroxide: Fe2O3 + 2NaOH = 2NaFeO2 + H2O.

(8)

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A. T. Volochko, K. B. Podbolotov, and A. A. Zhukova

during chemical reaction with silicon dioxide has a capacity to form readily melting eutectic. This problem has been described in detail in publications [10, 11]. In order to estimate the presence of iron in materials, prepared using ferrosilicon FS45 as a silicon-containing component, a study was made of their phase composition after heat treatment from 100 – 1100°C. Diffraction patterns showed presence in materials of the main crystalline phases, i.e., wollastonite and sodium silicate (Fig. 1) over the whole temperature range. With an increase in heat treatment temperature for materials there is a small reduction in the intensity of diffraction maxima for crystalline phases, which is probably connected with formation at high temperature of molten sodium silicate and partial dissolution of Fig. 1. XPA data for materials prepared at different temperature (shown on wollastonite. With a reduction in melt temperature, curves): l) wollastonite; n) calcium silicate. by not crystallizing it is transformed into glass, which as a rule is x-ray amorphic. Also, at 1000°C there is a transition of wollastonite (b-CaSiO3) into pseudo wollastonite (a-CaSiO3). Here no iron metal or its compounds were detected in diffraction patterns, which is connected with its small content that is below the XPA detection limit. Thus, XPA data (Fig. 1) are insufficient for evaluating presence or absence of iron in synthesized material. A study was made of the reaction of ferrosilicon with water glass, for which two specimens were prepared within whose composition there was only sodium water glass and ferrosilicon FS45. Specimens were heat treated at 100 and 800°C. Results of XPA for specimens are shown in Fig. 2. Analysis of x-ray patterns showed absence of elemental iron in synthesized specimens. It was established that firing of specimens at 800°C promotes polymorphic transformation of silica (on heating at 573°C b-quartz is converted into a-quartz), and also formation of a new phase, i.e., sodium ferrite (formation temperature 675 – 700°C). Thus, it has been established that in synthesized material there is no eleFig. 2. Dependence of intensity of diffraction maxima for crystalmental iron, or also FeO and Fe3O4, which on reaction with line phases on heat treatment temperature: a) 100°C; b ) 800°C. silicon form readily melting compounds. It is well known that the lower material density, the greater is its porosity, and the better are its heat insulation Occurrence of reactions (6) – (8) makes it impossible to properties under service temperature conditions for the proconfirm that there is no elemental iron in synthesized cetective structures of heat engineering units. As is well ramic material. Presence of iron in material has a negative efknown, each ceramic material is a combination of crystalline fect on many of its properties, among which are substance with cavities, i.e., pores. The pore volume, their thermophysical and strength properties. size, and nature of distribution, have a decisive effect on a As is well known, porous ceramic materials are used number of properties. Mechanical strength, thermal conducmainly as heat insulation of building and industrial objects tivity, chemical resistance, etc., depend on porosity. (www.izomat.ru). The thermal conductivity of these materiResults of measuring apparent density and ultimate als varies from 0.01 to 0.5 W/(m·K). Iron, being a metal, has strength in compression for specimens of the materials obhigh thermal conductivity, i.e., 86.5 W/(m·K), and therefore tained in relation to their silicon-containing component conits presence in the materials obtained has an unfavorable eftent are shown in Fig. 3. The least apparent density fect on their heat insulation capacity. A second reason for (450 – 550 kg/m3) applies to specimens containing silicon, which is explained by its greater activity compared with presence of iron being undesirable, is iron (III) oxide, which

Preparation of Porous Ceramic Materials Based on Wollastonite ferrosilicon, and correspondingly a greater degree of gas liberation reactions. With use of ferrosilicon there is an increase in specimen apparent density with a reduction in silicon concentration within it. The optimum is acknowledged as use of ferrosilicon grade FS75, which provides preparation of material with an apparent density of 600 – 700 kg/m3. An increase in the amount of silicon-containing component within a mix composition leads to a reduction in specimen density, although areas are noted with a sharp drop in density. With use of ferrosilicon grade FS75 the greatest change in density and strength of specimens is observed on introducing it in an amount of 5.0 – 7.5%, which is apparently connected with the optimum kinetic-mechanical factors. As noted previously, the greater the silicon content, the higher the rate of gas liberation; with a low reaction intensity and rate of gas liberation pore formation is made difficult by the liberation of a small amount of gas and partial filling pores with mix, which are in a pseudoplastic condition under the action of gravitational force, and correspondingly there is movement of pores with their emergence at the surface. This leads to formation of porosity only on removal of a significant amount of water, i.e., when the mix becomes stiff; also in this case there is an increase in local stresses due to significant pressure that arises in newly formed pores, and microcracks develop due to a lack of conditions for relaxation of stresses of mix plastic deformation. This is confirmed by the lower values of strength in compression. An increase in the intensity of reaction and rate of gas liberation leads to formation of a larger number of pores in the pseudoplastic stage, although there is more intense emergence of pores at a surface due to an increase in their total number. Thus, equilibrium is established between the forming and disappearing pores, and material porosity is almost unchanged. However, with considerable liberation of gas there is pore coalescence with disruption of continuity, which also leads to a reduction in material strength. Measurement of material thermal conductivity, obtained using ferrosilicon FS75, with a different content of it (Fig. 4), showed a reduction in material thermal conductivity with an increase in the amount of ferrosilicon within the composition of specimens, and the value of thermal conductivity correlates well with values of material density.

189

Fig. 3. Dependence of apparent density (———) and ultimate strength in compression (– – –) for specimens on silicon-containing component content: p) silicon; u) FS75; ´) FS45.

Fig. 4. Material thermal conductivity in relation for FS75 Ferrosilicon content (shown on curves, %).

Thus, the optimum amount of FS75 ferrosilicon, according to measurements of specimen apparent density, ultimate strength in compression and thermal conductivity, is 5.0 – 7.5%. The heat insulation material obtained has an apparent density of 610 – 660 kg/m3, high ultimate strength in compression of 6.2 – 7.2 MPa, and thermal conductivity (50 – 200°C) of 0.12 – 0.25 W/(m·K). An important factor for heat insulation materials, affecting the main operating properties, is porosity (pore size and shape). For synthesized materials based on wollastonite there is typically presence of coarse non-capillary pores, and also

Fig. 5. Microstructure of specimens with a different ferrosilicon content: a) 2.5%; b ) 5%; c) 10%.

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A. T. Volochko, K. B. Podbolotov, and A. A. Zhukova

TABLE 1. Physicomechanical Properties of Porous Ceramic Heat Insulation Materials Specimens prepared using Properties metallic silicon

FS75

FS45

Amount of silicon-containing component, %

2.5 – 5.0

5.0 – 7.5

7.5 – 10.0

Apparent density, kg/m3

490 – 550

610 – 660

1000 – 1100

Ultimate strength in compression, MPa

2.2 – 4.5

6.2 – 7.2

8.5 – 8.8

Open porosity, %

83 – 87

80 – 85

60 – 68

0.08 – 0.20

0.12 – 0.25

0.20 – 0.40

1100

1100

1100

Thermal conductivity, W/(m·K) Application temperature, °C

first order capillary pores (Fig. 5). Pore shape varies, i.e., both closed pores, which on joining with channel-forming, vermicular and branch-forming pores are transformed into open pores, and also blind pores. In the basic material, if volume is considered and not density, it has open porosity. A study of the microstructure of porous material also showed that with an increase in ferrosilicon content in the starting composition of specimens there is an increase not only in pore number, but also their dimensions. The main physicomechanical properties of heat insulation materials of optimum composition based on wollastonite are provided in Table 1. CONCLUSION As a result of these studies it has been established that: – there is no pure iron in the heat insulation material obtained after heat treatment at 100 and 800°C; – during material firing at 800°C iron (III) oxide, being a decomposition product of iron (III) hydroxide, forms sodium ferrite on reaction with alkali; – ferrosilicon grade FS75 is effective for replacing silicon, whose introduction in an amount of 5.0 – 7.5% promotes creates of the optimum kinetic and mechanical parameters for preparing material with low apparent density (610 – 660 kg/m3) with quite high ultimate strength in compression (6.2 – 7.2 MPa)’ – the thermal conductivity of the materials obtained is 0.12 – 0.25 W/(m·K), and the application temperature is 1100°C; – the microstructure of the material developed has pores of different shape, and their size varies from coarse non-capillary pores to first order capillary; – open porosity is typical for the material obtained.

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