INFLUENCE OF MECHANICAL GRINDING ON THE POZZOLANIC ACTIVITY OF RESIDUAL SUGARCANE BAGASSE ASH

Transcription

1 INFLUENCE OF MECHANICAL GRINDING ON THE POZZOLANIC ACTIVITY OF RESIDUAL SUGARCANE BAGASSE ASH Guilherme C. Cordeiro (1), Romildo D. Toledo Filho (1), Eduardo M. R. Fairbairn (1) Luis Marcelo M. Tavares (2) and Cristiano H. Oliveira (3) (1) Department of Civil Engineering/COPPE - Universidade Federal do Rio de Janeiro, Brazil. (2) Department of Metalurgical and Materials Engineering/COPPE - Universidade Federal do Rio de Janeiro, Brazil. (3) Centre for Mineral Technology - Ministério da Ciência e Tecnologia, Brazil. Abstract Since the beginning of the 20 th century sugarcane bagasse has been used as fuel in the boilers of the sugar factory. In 2003 approximately 95% of the sugarcane bagasse produced in Brazil was burned to generate energy resulting in about two million tons of residual ash. Due to the presence of amorphous silica in its chemical composition, this waste of the sugar industry can be used as a mineral admixture in cement paste, mortar and concrete. In order to enhance the pozzolanic activity of this residual ash it is necessary, however, to increase the fineness of the material. In this study mechanical grinding has been used to improve the pozzolanic activity of the sugarcane bagasse ash. The results of the study of the influence of grinding time on the sugarcane bagasse residual ash reactivity are presented. Its influence on particle size, specific surface (Blaine), scanning electronic microscopy, pozzolanic activity index, and energy consumption during grinding was analyzed so that the potential of the sugarcane bagasse ash use as a mineral admixture to pastes, mortars and concretes could be assessed. The Bond grindability test of the residual ash was performed and the work index determined at a test size of 325 mesh (45 m). The results demonstrate that the pozzolanic activity of the residual sugarcane bagasse ash can be significantly increased by mechanical grinding in a vibratory mill. Keywords: sugarcane bagasse ash, pozzolanic activity, mechanical grinding, ultrafine particle. 731

2 1. INTRODUCTION The sugar agroindustry is the oldest Brazilian economical activity and is related to the main historic events of the country. Brazil is currently the largest would producer of sugarcane, sugar and alcohol, besides being the largest sugar exporter in the world [1]. According to the Brazilian Institute of Statistics and Geography [2], in 2003 about 390 millions tons of sugarcane were produced and a production of 395 millions of tons is expected in The various stages involved in the production of sugar and alcohol are indicated in Figure 1. Basically, the production process includes the sugar harvest and transportation to the mill, followed by its washing and processing. The process for production of sugar or alcohol differentiates as the sugar stock is extracted and is treated in order to produce sugar and/or fermented to the alcohol production. Leaves Sugarcane WASHING Cane-washing water JUICE EXTRACTION Bagasse Filter tart JUICE TREATMENT JUICE TREATMENT SUGAR PRODUCTION FERMENTATION Yeast Sugar DISTILLATION Vinhoto Alcohol Figure 1: Production process of sugar and alcohol adapted from [3]. The major sugar-alcohol industry by-products, as seen in Figure 1, are cane-washing water, bagasse, leaves and ends, filter tart and yeast. The water used in the sugarcane washing, before grinding, can be reused in the biogas production and in the fertilized irrigation. The bagasse is used in the energy production (steam/electricity), fuel, hydrolysis, paper pulp, cellulose and wood veneer. The leaves and ends, besides having the same use as the bagasse, can be used as covering. Vinhoto is used as a fertilizer in the sugarcane plantations. Finally, the filter tart, a by-product that results from the stock clearing process in the sugar production, and the yeast, obtained after the stock fermentation, are used as fertilizers. As far as the by-products are concerned, the generation of electric energy, through combustion of bagasse, is significant. The usage of this waste is attractive, especially given the large tonnages of sugarcane processed in the sugar-alcohol sector, making it a significant quantity of generated bagasse; and heating value in the order of 7.74 MJ/kg at a moisture content of 50% [4]. Presently, about 95% of all bagasse produced in Brazil is burned in boilers for steam generation, while the other 5% are used as industrial raw material in the 732

3 production of paper, cellulose, alcohol, and wood veneer [5]. Figure 2 shows the composition of the Brazilian energy matrix [6]. Although it clearly demonstrates the importance of the hydraulic and oil-based energy in the Brazilian energy matrix ( 72%), it is important to note that the sugarcane contributes with 9.2% of the total. Figure 2: Brazilian energy matrix in 2001 [6]. The sugarcane bagasse consists of approximately 50% of cellulose, 25% of hemicellulose and 25% of lignin. Each ton of sugarcane generates approximately 26% of bagasse (at a moisture content of 50%) and 0.62% of residual ash [5]. Therefore, considering the production of 2003, about 2.3 million tons of residual ash become available in Brazil per year. The residue after combustion presents a chemical composition dominates by silicon dioxide (SiO 2 ). In spite of being a material of hard degradation and that presents few nutrients, the ash is used on the farms as a fertilizer in the sugarcane harvests [7]. In order to be used as a mineral admixture in concrete, the residual sugarcane ash must have appropriate physical and chemical properties [8]. Hernández et al. [9] emphasize the similarity between the chemical compositions of the rice husk residual ash, material studied by several researchers [10-11], and the residual bagasse ash. The wide particle size distribution presented by the residual bagasse ash suggests, however, the need for particle size reduction method in order to increase the specific surface and the reactivity of the ash. In this paper, the results of a study to evaluate the influence of grinding time on the sugarcane bagasse residual ash reactivity are presented. Results of particle size, specific surface (Blaine), scanning electronic microscopy, pozzolanic activity index, and energy consumption during grinding are presented and analyzed in order to demonstrate the potential use of the sugarcane bagasse ash as a mineral admixture to pastes, mortars and concretes. 2. MATERIALS The physical and chemical characteristics of the ordinary Portland cement used in this paper are presented in Table 1. The sugarcane bagasse ash was collected during the cleaning operation of a boiler operating in the Barcelos Sugar Factory, located in the city of São João da Barra (Brazil). Table 2 presents the chemical composition of the ash, determined by X-ray fluorescence spectroscopy. Mortars were manufactured using the Brazilian Standard Sand [12]. 733

4 Table 1: Chemical analyses and physical properties of ordinary Portland cement. Component Weight fraction (%) Mineral phase Weight fraction (%) SiO C 3 S Al 2 O C 2 S Fe 2 O C 3 A 2.33 CaO C 4 AF Na 2 O 0.16 K 2 O 0.40 SO Insoluble residue 0.18 Physical properties CaO free 1.16 Density 3170 kg/m 3 Available alkali (Na 2 O equivalent ) 0.42 Loss on ignition 1.05 Specific surface (Blaine) Fineness - passing # 325 (45 m) 308 m 2 /kg 8.40% Table 2: Chemical composition of residual sugarcane bagasse ash. Component Weight fraction (%) Component Weight fraction (%) SiO MnO 0.13 Al 2 O TiO Fe 2 O MgO 1.65 CaO 2.15 BaO < 0.16 Na 2 O 0.12 P 2 O K 2 O 3.46 Loss on ignition METHODOLOGY The batch grinding of the sugarcane bagasse ash was carried out in a vibratory mill (Aulmann & Beckschulte Mashininfabrik), which has 33-liter-steel cylindrical vase, with internal diameter of 19 cm. In every batch, 16.5 liters of cylindrical grinding media (13 mm x 13 mm) of alumina and 18 liters of sample were used. Grinding times were 8, 15, 30, 60, 120 and 240 minutes. The vibratory mill used consists of two cylinders connected to a system of masses which move away from center and produce a varying movement, of small amplitude, following a high frequency circular trajectory (Figure 3-a). Therefore, most of the impacts present fairly low magnitude and the motion of the grinding media is cascading [13]. Figure 3-b shows the mill used, which is installed in Mineral Technology Center (CETEM/MCT). The particle size composition was determined in each grinding condition with the aid of a laser particle analyzer (Malvern Martersizer ), using ethyl alcohol of analytical grade as the suspending medium and ultrasonic agitation during 60 seconds. The specific surface areas were determined using the Blaine method [14]. The pozzolanic activity of the products was evaluated through the definition of the pozolanic activity index with Portland cement (PI), according the NBR 5752 [14]. PI is defined as: f PI f cp cc.100 (1) 734

5 where: f cp : compressive strength of the mortar containing sugarcane bagasse ash; f cc : compressive strength of the OPC mortar. Supply Mass of work Shock absorber Vibration system Product Covering (a) Figure 3: Schematic illustration of a vibratory mill (a).vibratory mill of the Mineral Technology Center where the ash was ground (b). The mortars were prepared in the proportion 1 : 3 : 0.52 (binder : sand : water) by mass. Sugarcane bagasse ashes were used as replacement of Portland cement at alevel of 35% by volume and all mixtures presented flow consistence of 225 ± 5 mm, established by NBR 5752 [15]. In order to enable the microstructural analysis of the various mill products, the samples were sticked using a carbon conductor glue in brass-sample cases, followed by carbon-coating with the aid of the Jeol JEE-4X metalizer. After different grinding times, the products were analyzed with the scanning electronic microscope (Jeol JXA-A), with image detections by back-scattering eletrons, working with 20 kv and 39 mm work distance. The specific grinding energy was calculated using Bond s law, considering dry grinding condition in a ball mill, and open circuit operation. The Bond grindability test (used to determine Bond s work index) was performed with the 325 mesh (45 m) closing sieve in standard Bond ball mill (30.5 cm x 30.5 cm) following the standard procedure described in the NBR [16]. (b) 4. RESULTS AND DISCUSSION Figure 4 shows the particle size distributions obtained with the different times of grinding. With a 60-minute-grinding time, for example, the residual ash presents all particles with sizes smaller than 40 m. From this grinding time on, significant differences in the range of the particle size is reduced, changing the shape of the granulometric curve. 735

6 min Passing (wt., %) m in 30 min 60 min 120 min 240 min Without grinding Particle size ( m) Figure 4: Particle size distributions of sugarcane bagasse ashes for different grinding times. As the particle size distributions are continuous two indices, D 10 and D 80, are used to characterize the granulometric distribution of the ashes for the used grinding times. D 10 is the material particle size at which 10% of the weight is finer. D 80 is the corresponding value at 80% finer. Figure 5 shows the variation of the characteristic indices with the grinding times. The results indicate that after 60 minutes of grinding there is not a significant reduction in the particle sizes since the vibratory mill was not efficient enough to ground the residual ash to size smaller than 0.3 m. This happens because sub-micron particles are naturally more resistant and tend to deform plastically instead of breaking in smaller sizes. Additionally, the probability of the fine particles being captured by the grinding media is very low [17] D10 D 10 D80 D Particle size ( m) Grinding time (minutes) Figure 5: Relation between curve characteristic criteria (D 10 and D 80 ) and grinding times. Figure 6 shows the morphology of the residual ashes after being ground for different times. The grinding process did not change significantly the particle shape. Contamination of the ash by quartz particles is noted and this can be attributed to the presence of sand which is not totally removed during the sugarcane washing

7 Moagem min 15 (a) (b) Moagem min (c) 240 (d) (e) Figure 6: SEM images of the sugarcane residual bagasse ashes with different grinding times: 8 (a), 15 (b), 30 (c), 60 (d), 120 (e) and 240 minutes (f). Figure 7 shows the effect of the grinding time on the specific surface area of the residual sugarcane bagasse ash. A significant increase of the ash specific surface area can be observed with the increase of grinding time. The linear relationship (up to 120 minutes of grinding) suggests the validity of the Rittinger s law of comminution [18]. This law establishes that the increase in the specific surface area due to the comminution process is directly proportional to the mechanical work used in this operation. A non-linear behaviour is observed after 120 minutes of grinding and this can also be attributed to the difficulty of the sub-micron particles being breaked by the grinding media. (f) 737

8 Specific surface - Blaine (m 2 /kg) R 2 = Grinding time (minutes) 0 Figure 7: Effect of grinding time on the specific surface area of sugarcane ash. The results of the compression tests carried out in mortars mixes of same workability are presented in Table 3. The pozzolanic activity index, calculated using of Equation (1) are presented in Figure 8. The results indicate that after 15 minutes of grinding the PI is already higher than the minimum value established by Brazilian standard NBR [8]. The ashes produced by 60 and 120 minutes of the grinding presented pozzolanic index of, respectively, 89% and 100%. This result confirms the importance of the specific surface area on the activity of pozzolanic materials. An increase of the grinding time from 120 min to 240 min did not improve a great benefit on the pozzolanic index (an increase of 3%). From the correlating obtained in the present study it is possible to estimate the grinding time and energy consumption (for a ball mill in industrial scale) necessary to obtain an ash with a specific granulometric characteristic index D 80. An association between grinding time, particle size and specific energy (calculated from Bond s law) is presented in Figure 9. In order, for example, to obtain an ash with D 80 equal to 15 m, it would be necessary the ground the ash for 60 minutes in the vibratory mill (Figure 9-a). The energy consumption in an industrial scale, with a ball mill operating continually in open circuit, can be estimated from Figure 9-b. For the given example, a specific energy consumption of about 245 kw.h/t is obtained. It is important to point out that the value of Bond s work index determined for the sugarcane residual ash, which was used in the calculation of the specific energy consumption was kw.h/t. Table 3: Compressive strength and flow index for the mortars studied. Mortar mix Mean* compressive Standard deviation Flow index strength (MPa) (MPa) (mm) Control Ash Ash Ash Ash Ash Ash * Four specimens. 738

9 Pozzolanic activity index (%) NBR PI 60 SS Grinding time (minutes) Specific surface - Blaine (m 2 /kg). Figure 8: Effect of grinding time on the pozzolanic activity index (PI) and specific surface area (SS) values of residual sugarcane bagasse ash. The broken line indicates the minimum PI value established by the NBR [8] Particle size, D 80 ( m) Particle size, D 80 ( m) Grinding time (minutes) Specific energy (kw.h/t) 1 (a) Figure 9: Energy consumption for grinding of the bagasse ash according to Bond s law. Correlation proposal between vibratory mill grinding (a) and ball mill in idustrial scale (b). (b) 4. CONCLUDING REMARKS The results obtained in this investigation indicate that, after an adequate mechanical grinding, the residual sugarcane residual ash can be used as a pozzolanic admixture in concrete. The minimum value of pozzolanic activity index, established by Brazilian standard NBR [8], was reached after 15 minutes of grinding. Grinding the ash for 120 made possible to obtain a pozzolan with an activity index of 100%. An increase on the grinding time from 120 min to 240 min did not benefit significantly activity of the pozzolan because of the difficulty in grinding sub-micron particles. The results also indicate that the PI increases with the increase of the specific surface area of the sugarcane ash. 739

10 The sugarcane bagasse ash had an experimental Bond s work index of kw.h/t. An association between grinding time, particle size and specific energy (calculated from Bond s law) permitted an estimation of the energy consumption necessary to obtain an ash with a particular granulometric characteristic index D 80. ACKNOWLEDGMENTS The authors would like to acknowledge the Brazilian Agencies, CNPq, CAPES and FAPERJ, for the financial support. REFERENCES [1] Azevedo, H.J., 'Uma análise da cadeia produtiva de cana-de-açúcar na Região Norte Fluminense', 1nd Edn (Rio de Janeiro, Consórcio Universitário de Pesquisa da Região Norte Fluminense, 2002). [2] IBGE Statistics and Geography Barzilian Institute, 'Levantamento sistemático da produção agrícola'. Online: WWW. URL: 23/03/2004. [3] Moreira, J.R. and Goldemberg, J., 'The alcohol program', Energy Policy 27 (1999) [4] Coelho, S.T., 'Mecanismos para implementação da co-geração de eletricidade a partir de biomassa. Um modelo para o Estado de São Paulo', Doctoral Thesis (São Paulo, Universidade de São Paulo, 1999). [5] FIESP/CIESP, 'Ampliação da oferta de energia através da biomassa (bagaço da cana-de-açúcar)', 1nd End (São Paulo, FIESP/CIESP, 2001). [6] National Energetic Balance BEN, (Brasília, Ministério de Minas e Energia, 2001). [7] Manhães, M.S., 'Adubação, correção do solo e uso de resíduos da agroindústria', 1nd Edn (Campos dos Goytacazes, UFRRJ, 1999). [8] Technical Standards Brazilian Association, 'Materiais pozolânicos: NBR 12653' (Rio de Janeiro, 1992). [9] Hernández, J.F.M., Middeendorf, B., Gehrke, M. and Budelmann, H., 'Use of wastes of the sugar industry as pozzolana in lime-pozzolana binders: study of the reaction', Cement and Concrete Research 28 (1998) [10] Mehta, P.K., 'Properties of blended cements made from rice husk ash', ACI Journal 74 (40) (1977) [11] Malhotra, V.M. and Mehta, P.K., 'Pozzolanic and cementitious materials', 1nd Edn (Amsterdam, Gordon and Breach Publishers, 1996). [12] Technical Standards Brazilian Association, Areia normal para ensaio de cimento: NBR 7214' (Rio de Janeiro, 1982). [13] Wellenkamp, F.-J., 'Moagens fina e ultrafina de minerais industriais: uma revisão', 1nd Edn (Rio de Janeiro, CETEM/MCT, 1999). [14] Technical Standards Brazilian Association, 'Cimento Portland - Determinação da finura pelo método de permeabilidade ao ar (Método Blaine): NBR NM 76' (Rio de Janeiro, 1998). [15] 'Materiais pozolânicos - Determinação da atividade pozolânica com cimento Portland - Índice de atividade pozolânica com cimento: NBR 5752' (Rio de Janeiro, 1992). [16] 'Moinho de bolas - Determinação do índice de trabalho: NBR 11376' (Rio de Janeiro, 1990). [17] Austin, L.G. and Concha, F., 'Diseño y simulación de circuitos de molienda y clasificación', 1nd Edn (Concepción, CYTED, 1994). [18] Rumpf, H., 'Physical aspects of comminution and new formulation of a law of comminution', Powder Technology 7 (1973)