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Thermal expansion of slate wastes
Minerals Engineering 19 (2006) 518–520 This article is also available online at: www.elsevier.com/locate/mineng
Technical note
Thermal expansion of slate wastes M.E.M.C. Silva a, A.E.C. Peres a
b,*
Centro Tecnolo´gico de Minas Gerais, Avenida Jose´ Caˆndido da Silveira, 2000, 31170-000 Belo Horizonte, MG, Brazil b Universidade Federal de Minas Gerais, Rua Espirito Santo, 35/206, 30160-030 Belo Horizonte, MG, Brazil Received 25 July 2005; accepted 11 October 2005 Available online 28 November 2005
Abstract Among the technological routes for recycling mining and beneficiation slate wastes, thermal expansion seems particularly attractive, yielding products adequate for use in civil construction, especially as pozzolanic material for cement manufacture. The present investigation aims at analysing the properties of slate before and after the thermal expansion, searching for the reasons of expansion and the variables that affect the characteristics and properties of the products. Results of standard test procedures recommended by the construction industry are presented and discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Slate waste processing; Thermal expansion; Industrial minerals
1. Introduction
2. Experimental
Slate mining and beneficiation wastes are inadequately disposed in piles. These wastes represent a significant amount of material with potential for utilisation. The proposed alternative is treating this material by thermal expansion to generate products for utilisation in the cement industry. Studies were performed searching for a correlation among mineralogical, petrographic and geochemical properties of slate with those of the thermal expansion products with the aim to shed light onto the causes of the expansion and the variables that affect the characteristics and properties of the products. Natural slate wastes do not possess pozzolanic activity. Thermally expanded wastes, on the other hand, may replace partially the Portland cement clinker as a pozzolanic additive.
2.1. Materials
*
Corresponding author. Tel.: +55 313 238 1717; fax: +55 313 238 1815. E-mail address: [email protected] (A.E.C. Peres).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.008
The following materials were used: (i) grey slate wastes (from mining and cutting activities) from the Slate Province of Minas Gerais (Grossi Sad et al., 1998); (ii) Portland cement clinker and gypsum, used in tests of axial compression resistance for comparison with slate as pozzolanic additive; (iii) sand, for composing the mortar mix.
2.2. Thermal expansion A literature review suggested the use of a rotary tubular kiln in the thermal process of expansive clays and slates (Santos, 1992; Cinasita, 2001; Stalite, 2001; ESCSI, 2001). An electrically heated kiln was manufactured. The option for the electrically heated kiln for the laboratory experiments was based on a better operation control, especially regarding temperature, and a lower capital cost. At pilot and industrial
M.E.M.C. Silva, A.E.C. Peres / Minerals Engineering 19 (2006) 518–520
scales this type of kiln is not economically viable. Heating is achieved via resistive elements of silicon carbide, disposed in two zones. The temperature control is independent for each zone, allowing operation up to 1400 °C. The wastes of slate were cut in squares or rectangles (1– 4 cm) for preliminary thermal expansion tests. Later the samples were comminuted and screened. The fraction 12.7 mm + 6.35 mm was selected, for the initial expectation was meeting the specifications for lightweight aggregates. For thermal expansion tests the size range was not relevant and the selection was based on the dimensions of the kiln. The main variables that may influence the thermal expansion process are: type of the kiln; feed rate, size distribution, heating rate, time and temperature of calcination, type of slate. Only one type of kiln was utilised, allowing a feed of 30 g per batch. The heating rate is a relevant variable. A high heating rate may cause decrepitation, due to the high rate of water leaving the particle, resulting in aggregates with low mechanical resistance (Martins and Lima, 1996). In the preliminary tests, heating rates of 20, 10, 5 °C/min and calcination temperatures of 1100 and 1300 °C were utilised, considering the operation limitations of the kiln, the thermal behaviour of the slate samples identified by thermo-gravimetric and thermo-differential analyses, as well as references to other studies (Martins and Lima, 1996) and industrial application (Stalite, 2001). The kiln consists of two adjacent heating zones. The sample was heated up to 600 °C in the cool zone, then the slope of the kiln was changed and the sample was moved to the hot zone. After the preliminary experiments, the heating rate was set at 5 °C/min up to 600 °C, then at 15 °C/min up to the calcination temperature (1170 and 1190 °C). Calcination time at each calcination temperature was set at 5 and 10 min. The thermal expansion products were characterised via (i) X-ray diffraction (degree of remaining crystallinity); (ii) chemical analyses (expressed as oxides): FeO (wet chemistry, volummetry with K2Cr2O7); Al2O3, CaO, Fe2O3, MgO, MnO (fusion—ICP); Na2O, K2O (microwaves—ICP); loss on ignition—LOI (calcination at 950 °C ± 50 °C 1 h); (iii) microscopic analyses (size and dimensions of the pores, preferential direction of the expansion and degree of homogeneity of the phases produced by calcination); (iv) determination of the specific mass, water absorption and mechanical resistance.
519
tially replaced (35% by volume) by the material under investigation. The index of pozzolanic activity is the ratio between the resistance to axial compression after 28 days of the two mortars: (1) reference mortar containing the blend clinker + gypsum; (2) 35% in volume of the reference blend replaced by the presumed pozzolanic material. Another method for determination of the pozzolanic activity is the chemical method (NBR 5753, 1991). Wastes of grey slate (the most abundant type) were selected for the determination of pozzolanic activity, at its natural condition and after expansion. The samples were ground to maximum 34% retained in 0.045 mm. 2.4. Cement mortar method Specimens were prepared with the standard mix (clinker + gypsum + normal sand) and with slate replacing the compounds clinker and gypsum (35% in volume). The specimens were kept under controlled temperature (38 °C ± 2 °C) for 28 days. 2.5. Chemical method According to this method the pozzolanic activity is evaluated by comparing the amount of Ca(OH)2 present in the liquid phase in contact with hydrated cement with the amount of Ca(OH)2 required for the saturation of the medium at the same alkalinity. 3. Results and discussion 3.1. Characterisation studies The major mineralogical components of the slate, according to X-ray diffraction, are: white sericite mica, quartz, chlorite, feldspar, and carbonates. Minor components are epidote, turmaline, zircon, apatite, biotite, and opaque minerals. The presence of significant amounts of chlorite, mica, carbonates, hematite and pyrite, affects the expansion of slates, due to the role of these minerals in the characteristic structure and texture. Chemically, the major species detected were SiO2, Al2O3, FeO, Fe2O3, K2O, MgO, and Na2O. The composition varies widely even for slates from the same deposit, rendering it difficult to correlate the chemical composition with thermal expansion. Nevertheless, the average composition is within the range predicted for thermal expansion. The average value obtained for the specific mass was 2.7 and low indexes of water absorption were determined (<0.2%).
2.3. Characterisation of slate as a mineral additive in cement manufacture
3.2. Thermal expansion process
Pozzolanic activities with cement and lime are standardised in Brazil (NBR 5752, 1992). Cement or lime is par-
The calcination temperature affects the degree of expansion of the slate. The products achieved at 1250 °C present
520
M.E.M.C. Silva, A.E.C. Peres / Minerals Engineering 19 (2006) 518–520
lower specific mass and mechanical resistance than those at 1180 °C. The thermal expansion products are predominantly amorphous with only quartz and pseudo-spinel with a low crystallinity are present as crystalline phases, according to X-ray diffraction, indicating significant structure change with respect to the natural slate. Regarding chemical composition, the higher expansion indexes of slates are associated with higher levels of loss on ignition, due to CO2 and H2O losses. The LOI index decreases from 3.5% in the natural slate to 0.1% after expansion, confirming the importance of the presence of structural water in the slate, as well as the evolution of CO2 in the expansion. The average value obtained for the specific mass was 0.90 for thermal expansion products and low values for water absorption index were determined, both before and after thermal expansion. 3.3. Characterisation of slate as mineral additive Specifications from NBR 12653 (1992) and from ASTM C 618 (1997) for the chemical composition of pozzolanic class N materials are compared with the average values obtained for the natural and the product from the thermal expansion of grey slate: SiO2 + Al2O3 + Fe2O3 (%min): 70 (NBR & ASTM); 81.0 (natural); 85.9 (expanded). SO3 (%max): 4.0 (NBR & ASTM); <125 ppm (natural); <125 ppm (expanded). Moisture (%max): 3.0 (NBR & ASTM); <1 (natural); <1 (expanded). LOI (%max): 10.0 (NBR & ASTM); 3.43 (natural); 0.01 (expanded). Alkalis as total Na2O (%max): 1.5 (NBR & ASTM); 1.93 (natural); 1.70 (expanded). Results of pozzolanic activity (chemical method) related to natural slates did not confirm the pozzolanicity of the mortars. On the other hand, the results confirmed pozzolanic activity of the sample with calcinated slate. Results of pozzolanic activity (mortar method), also confirmed pozzolanic activity of the sample with calcinated slate. Thermally expanded slate presents pozzolanic activity level higher than the standard minimum limit and similar to that for the reference mortar. The pozzolanicity of the thermally treated slate is not dependent on its chemical composition, but whether silica present in it (and to a lesser extent also the alumina) is in an amorphous state that can be activated to react easily with the Ca(OH)2 released during cement hydration to form additional cementitious C–S–H compounds to contribute to the strength development of the cement.
4. Conclusions The calcination temperature is relevant regarding the thermal expansion parameters: expansion degree, specific mass and mechanical resistance of the products. The expansion of slates occurs in the direction perpendicular to the cleavage, as observed in the specimens produced, indicating a close correlation between structure, mineralogical composition and thermal expansion. It is believed that the presence of filosilicates, parallel to the cleavage plane, presenting inserted layers of structural water, plays a relevant role in the thermal expansion. The sequence of structure water between the plaques also plays a relevant role regarding the thermal expansion, due to the release of water present in the slate structure. The chemical composition of the thermal expansion products, as well as their low crystallinity degree, are within the specifications for use as pozzolanic materials. The results of pozzolanic activity, obtained by the chemical method, were confirmed by the results of mechanical resistance of mortars, showing that the products of slate thermal expansion present pozzolanic activity. Products from thermal expansion of slates, due to their pozzolanic character, represent a technically viable alternative for the use of wastes from mining and beneficiation of slates. Natural slate does not present pozzolanic activity but its chemical composition is compatible with its use as mineral addition to clinker. References ASTM C 618, 1997. American Society for Testing and Material—ASTM C 618. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete. Philadelphia. Cinasita, 2001. (Cinasita S. A—Indu´stria e Come´rcio), Sa˜o Paulo. Available from: http://www.cinasita.com.br/ (in Portuguese). ESCSI, 2001. Expanded Shale, Clay & Slate Institute (ESCSI). Available from: http://www.escsi.org/. Grossi Sad, J.H., Chiodi Filho, C., Chiodi, D.K., 1998. Scenario of slates in Minas Gerais state, Brazil. COMIG, 2v., Belo Horizonte (in Portuguese). Martins, J., Lima, N.P., 1996. Utilisation of sized slate wastes as light aggregates in civil construction. In: 51° Congresso Anual da ABM, Porto Alegre, Proceedings (in Portuguese). NBR 5752, 1992. Associac¸a˜o Brasileira de Normas Te´cnicas. Pozzolanic materials. Rio de Janeiro (in Portuguese). NBR 5753, 1991. Associac¸a˜o Brasileira de Normas Te´cnicas. Cements. Rio de Janeiro (in Portuguese). NBR 12653, 1992. Associac¸a˜o Brasileira de Normas Te´cnicas. Pozzolanic materials. Rio de Janeiro (in Portuguese). Santos, P.S., 1992. Science and Technology of Clays, second ed. Edgard Blu¨cher Ltda, Sa˜o Paulo, pp. 242–302 and 438–467 (in Portuguese). Stalite, Carolina Stalite Company, 2001. Performance Lightweigh Aggregate. Available from:
Technical note
Thermal expansion of slate wastes M.E.M.C. Silva a, A.E.C. Peres a
b,*
Centro Tecnolo´gico de Minas Gerais, Avenida Jose´ Caˆndido da Silveira, 2000, 31170-000 Belo Horizonte, MG, Brazil b Universidade Federal de Minas Gerais, Rua Espirito Santo, 35/206, 30160-030 Belo Horizonte, MG, Brazil Received 25 July 2005; accepted 11 October 2005 Available online 28 November 2005
Abstract Among the technological routes for recycling mining and beneficiation slate wastes, thermal expansion seems particularly attractive, yielding products adequate for use in civil construction, especially as pozzolanic material for cement manufacture. The present investigation aims at analysing the properties of slate before and after the thermal expansion, searching for the reasons of expansion and the variables that affect the characteristics and properties of the products. Results of standard test procedures recommended by the construction industry are presented and discussed. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Slate waste processing; Thermal expansion; Industrial minerals
1. Introduction
2. Experimental
Slate mining and beneficiation wastes are inadequately disposed in piles. These wastes represent a significant amount of material with potential for utilisation. The proposed alternative is treating this material by thermal expansion to generate products for utilisation in the cement industry. Studies were performed searching for a correlation among mineralogical, petrographic and geochemical properties of slate with those of the thermal expansion products with the aim to shed light onto the causes of the expansion and the variables that affect the characteristics and properties of the products. Natural slate wastes do not possess pozzolanic activity. Thermally expanded wastes, on the other hand, may replace partially the Portland cement clinker as a pozzolanic additive.
2.1. Materials
*
Corresponding author. Tel.: +55 313 238 1717; fax: +55 313 238 1815. E-mail address: [email protected] (A.E.C. Peres).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.008
The following materials were used: (i) grey slate wastes (from mining and cutting activities) from the Slate Province of Minas Gerais (Grossi Sad et al., 1998); (ii) Portland cement clinker and gypsum, used in tests of axial compression resistance for comparison with slate as pozzolanic additive; (iii) sand, for composing the mortar mix.
2.2. Thermal expansion A literature review suggested the use of a rotary tubular kiln in the thermal process of expansive clays and slates (Santos, 1992; Cinasita, 2001; Stalite, 2001; ESCSI, 2001). An electrically heated kiln was manufactured. The option for the electrically heated kiln for the laboratory experiments was based on a better operation control, especially regarding temperature, and a lower capital cost. At pilot and industrial
M.E.M.C. Silva, A.E.C. Peres / Minerals Engineering 19 (2006) 518–520
scales this type of kiln is not economically viable. Heating is achieved via resistive elements of silicon carbide, disposed in two zones. The temperature control is independent for each zone, allowing operation up to 1400 °C. The wastes of slate were cut in squares or rectangles (1– 4 cm) for preliminary thermal expansion tests. Later the samples were comminuted and screened. The fraction 12.7 mm + 6.35 mm was selected, for the initial expectation was meeting the specifications for lightweight aggregates. For thermal expansion tests the size range was not relevant and the selection was based on the dimensions of the kiln. The main variables that may influence the thermal expansion process are: type of the kiln; feed rate, size distribution, heating rate, time and temperature of calcination, type of slate. Only one type of kiln was utilised, allowing a feed of 30 g per batch. The heating rate is a relevant variable. A high heating rate may cause decrepitation, due to the high rate of water leaving the particle, resulting in aggregates with low mechanical resistance (Martins and Lima, 1996). In the preliminary tests, heating rates of 20, 10, 5 °C/min and calcination temperatures of 1100 and 1300 °C were utilised, considering the operation limitations of the kiln, the thermal behaviour of the slate samples identified by thermo-gravimetric and thermo-differential analyses, as well as references to other studies (Martins and Lima, 1996) and industrial application (Stalite, 2001). The kiln consists of two adjacent heating zones. The sample was heated up to 600 °C in the cool zone, then the slope of the kiln was changed and the sample was moved to the hot zone. After the preliminary experiments, the heating rate was set at 5 °C/min up to 600 °C, then at 15 °C/min up to the calcination temperature (1170 and 1190 °C). Calcination time at each calcination temperature was set at 5 and 10 min. The thermal expansion products were characterised via (i) X-ray diffraction (degree of remaining crystallinity); (ii) chemical analyses (expressed as oxides): FeO (wet chemistry, volummetry with K2Cr2O7); Al2O3, CaO, Fe2O3, MgO, MnO (fusion—ICP); Na2O, K2O (microwaves—ICP); loss on ignition—LOI (calcination at 950 °C ± 50 °C 1 h); (iii) microscopic analyses (size and dimensions of the pores, preferential direction of the expansion and degree of homogeneity of the phases produced by calcination); (iv) determination of the specific mass, water absorption and mechanical resistance.
519
tially replaced (35% by volume) by the material under investigation. The index of pozzolanic activity is the ratio between the resistance to axial compression after 28 days of the two mortars: (1) reference mortar containing the blend clinker + gypsum; (2) 35% in volume of the reference blend replaced by the presumed pozzolanic material. Another method for determination of the pozzolanic activity is the chemical method (NBR 5753, 1991). Wastes of grey slate (the most abundant type) were selected for the determination of pozzolanic activity, at its natural condition and after expansion. The samples were ground to maximum 34% retained in 0.045 mm. 2.4. Cement mortar method Specimens were prepared with the standard mix (clinker + gypsum + normal sand) and with slate replacing the compounds clinker and gypsum (35% in volume). The specimens were kept under controlled temperature (38 °C ± 2 °C) for 28 days. 2.5. Chemical method According to this method the pozzolanic activity is evaluated by comparing the amount of Ca(OH)2 present in the liquid phase in contact with hydrated cement with the amount of Ca(OH)2 required for the saturation of the medium at the same alkalinity. 3. Results and discussion 3.1. Characterisation studies The major mineralogical components of the slate, according to X-ray diffraction, are: white sericite mica, quartz, chlorite, feldspar, and carbonates. Minor components are epidote, turmaline, zircon, apatite, biotite, and opaque minerals. The presence of significant amounts of chlorite, mica, carbonates, hematite and pyrite, affects the expansion of slates, due to the role of these minerals in the characteristic structure and texture. Chemically, the major species detected were SiO2, Al2O3, FeO, Fe2O3, K2O, MgO, and Na2O. The composition varies widely even for slates from the same deposit, rendering it difficult to correlate the chemical composition with thermal expansion. Nevertheless, the average composition is within the range predicted for thermal expansion. The average value obtained for the specific mass was 2.7 and low indexes of water absorption were determined (<0.2%).
2.3. Characterisation of slate as a mineral additive in cement manufacture
3.2. Thermal expansion process
Pozzolanic activities with cement and lime are standardised in Brazil (NBR 5752, 1992). Cement or lime is par-
The calcination temperature affects the degree of expansion of the slate. The products achieved at 1250 °C present
520
M.E.M.C. Silva, A.E.C. Peres / Minerals Engineering 19 (2006) 518–520
lower specific mass and mechanical resistance than those at 1180 °C. The thermal expansion products are predominantly amorphous with only quartz and pseudo-spinel with a low crystallinity are present as crystalline phases, according to X-ray diffraction, indicating significant structure change with respect to the natural slate. Regarding chemical composition, the higher expansion indexes of slates are associated with higher levels of loss on ignition, due to CO2 and H2O losses. The LOI index decreases from 3.5% in the natural slate to 0.1% after expansion, confirming the importance of the presence of structural water in the slate, as well as the evolution of CO2 in the expansion. The average value obtained for the specific mass was 0.90 for thermal expansion products and low values for water absorption index were determined, both before and after thermal expansion. 3.3. Characterisation of slate as mineral additive Specifications from NBR 12653 (1992) and from ASTM C 618 (1997) for the chemical composition of pozzolanic class N materials are compared with the average values obtained for the natural and the product from the thermal expansion of grey slate: SiO2 + Al2O3 + Fe2O3 (%min): 70 (NBR & ASTM); 81.0 (natural); 85.9 (expanded). SO3 (%max): 4.0 (NBR & ASTM); <125 ppm (natural); <125 ppm (expanded). Moisture (%max): 3.0 (NBR & ASTM); <1 (natural); <1 (expanded). LOI (%max): 10.0 (NBR & ASTM); 3.43 (natural); 0.01 (expanded). Alkalis as total Na2O (%max): 1.5 (NBR & ASTM); 1.93 (natural); 1.70 (expanded). Results of pozzolanic activity (chemical method) related to natural slates did not confirm the pozzolanicity of the mortars. On the other hand, the results confirmed pozzolanic activity of the sample with calcinated slate. Results of pozzolanic activity (mortar method), also confirmed pozzolanic activity of the sample with calcinated slate. Thermally expanded slate presents pozzolanic activity level higher than the standard minimum limit and similar to that for the reference mortar. The pozzolanicity of the thermally treated slate is not dependent on its chemical composition, but whether silica present in it (and to a lesser extent also the alumina) is in an amorphous state that can be activated to react easily with the Ca(OH)2 released during cement hydration to form additional cementitious C–S–H compounds to contribute to the strength development of the cement.
4. Conclusions The calcination temperature is relevant regarding the thermal expansion parameters: expansion degree, specific mass and mechanical resistance of the products. The expansion of slates occurs in the direction perpendicular to the cleavage, as observed in the specimens produced, indicating a close correlation between structure, mineralogical composition and thermal expansion. It is believed that the presence of filosilicates, parallel to the cleavage plane, presenting inserted layers of structural water, plays a relevant role in the thermal expansion. The sequence of structure water between the plaques also plays a relevant role regarding the thermal expansion, due to the release of water present in the slate structure. The chemical composition of the thermal expansion products, as well as their low crystallinity degree, are within the specifications for use as pozzolanic materials. The results of pozzolanic activity, obtained by the chemical method, were confirmed by the results of mechanical resistance of mortars, showing that the products of slate thermal expansion present pozzolanic activity. Products from thermal expansion of slates, due to their pozzolanic character, represent a technically viable alternative for the use of wastes from mining and beneficiation of slates. Natural slate does not present pozzolanic activity but its chemical composition is compatible with its use as mineral addition to clinker. References ASTM C 618, 1997. American Society for Testing and Material—ASTM C 618. Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete. Philadelphia. Cinasita, 2001. (Cinasita S. A—Indu´stria e Come´rcio), Sa˜o Paulo. Available from: http://www.cinasita.com.br/ (in Portuguese). ESCSI, 2001. Expanded Shale, Clay & Slate Institute (ESCSI). Available from: http://www.escsi.org/. Grossi Sad, J.H., Chiodi Filho, C., Chiodi, D.K., 1998. Scenario of slates in Minas Gerais state, Brazil. COMIG, 2v., Belo Horizonte (in Portuguese). Martins, J., Lima, N.P., 1996. Utilisation of sized slate wastes as light aggregates in civil construction. In: 51° Congresso Anual da ABM, Porto Alegre, Proceedings (in Portuguese). NBR 5752, 1992. Associac¸a˜o Brasileira de Normas Te´cnicas. Pozzolanic materials. Rio de Janeiro (in Portuguese). NBR 5753, 1991. Associac¸a˜o Brasileira de Normas Te´cnicas. Cements. Rio de Janeiro (in Portuguese). NBR 12653, 1992. Associac¸a˜o Brasileira de Normas Te´cnicas. Pozzolanic materials. Rio de Janeiro (in Portuguese). Santos, P.S., 1992. Science and Technology of Clays, second ed. Edgard Blu¨cher Ltda, Sa˜o Paulo, pp. 242–302 and 438–467 (in Portuguese). Stalite, Carolina Stalite Company, 2001. Performance Lightweigh Aggregate. Available from: