STUDIES ON SINTERING OF MOLDED GRANITE BODIES

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1 STUDIES ON SINTERING OF MOLDED GRANITE BODIES V.X. de Lima Filho, P. F Blazdell *, R. E. F. Q Nogueira and R. Braga Universidade Federal do Ceara Centro de Tecnologia, Bloco 714, Caixa Postal 12144, Fortaleza, CE *Author to whom correspondence should be addressed Abstract Production of hazardous waste as a result of industrial production is without a doubt one of today's most serious environmental problems. This study describes a potential use for this waste powder as a substitute for conventional engineering ceramics. Granite powders which are by-products from quarrying ornamental stone causes significant environmental damage. In order to assess the ability of the powers to be sintered into a compact final form samples were pressed and then sintering was performed in air at different temperatures. Upon optimisation of the sintering process simple shapes were produced by injection moulding and sintered to near full density. Initial results obtained indicate that the low-pressure injection moulding of granite powders can be used to produce components that can subsequently be sintered to near full density. The potential of using granite powder as a source of feedstock for various ceramic processing routes demonstrated.. Key Words: granite, injection moulding, sintering, ceramic processing, thread guides CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10401

2 1. Introduction World consumption of granite-type materials used for construction purposes passed 400 million tons per year in the early 1990's and the use is not anticipated to decline in the near future. A significant proportion (approximately 80%) of this material is employed in the building industry. Brazilian production of granite amounts to some 900,000 tonnes per year. The estimated waste products, which are normally in the form of powders, produced as a direct result of these manufacturing processes, are approximately 540,000 tonnes per year (1). This waste of material is a serious obstacle for the full-scale industrial development and is currently discarded often at significant risk of major environmental damage. Accumulation of granite powder presents not only a health hazard but also a potential environmental problem as it can pollute soil and rivers. Reducing the impact on the environment is therefore certainly a major concern to the granite industry (2), but is one area which until recently has received little interest, or funding. Using the residues, as an alternative source of income is a very attractive option for the industry, which is currently under going serious financial crises. This is particularly true in North-eastern Brazil. However, little work has so far been published on the use of these residue powders and their full potential lies largely untapped. Our group, in a previous publication, has shown conclusively that these waste powders can be used as raw material for other manufacturing processes (3). The use of the so-called traditional ceramics (bricks, tiles, pottery, and china) is well established and well documented. Generally traditional ceramics, which includes materials such as building bricks, construction pipes and various kinds of tiles are characterised by low cost manufacture, inexpensive raw materials and a low cost for the resultant product. The use of such traditional ceramics differs from the use of advanced or engineering ceramics in that the end product is often used under more demanding conditions and thus the final product is more expensive. However, both types of ceramic confer a high end-user value in terms of cost-benefits. It is for this reason that ceramic research continues to be of major interest both at academic and corporate level. Table 1 gives a brief comparison between traditional and advanced ceramics. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10402

3 The main objective of this work was to demonstrate the potential for the use of granite powder residues as a raw material for injection moulding further papers will report the properties of the components produced. Attention in particular was focused on components that do not demand high chemical purity, such as non-critical structural parts and threadguides. Thread guides are of particular interest as they are small, relatively low quality components, which are used in significant volume in the textile industry. One of the key properties required from a thread guide is good resistance to wear and although the hardness of the granite, as given by Moh's hardness, is slightly less then aluminia (about 7 compared to 9 for commercially pure aluminia) this small reduction in resistance to wear does not preclude the use of granite thread guides. The ability to manufacture these components from a waste product would be of significant financial interest and would allow many textile manufacturers to produce textiles at significantly reduced costs due to the granite-powder thread guides being cheaper to produce than conventional ceramic guides. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10403

4 2. Experimental Procedure 2.1. Material selection: Powder characteristics have a great influence on the final properties. In general, the desired characteristics for a ceramic powder suitable for injection moulding are: small particle size (µm), narrow particle size distribution, equiaxial shape and chemical purity (5). The granite powder used in this study is composed mainly of low quartz (connected chains of SiO 2 tetrahedra,) which has a density of 2650 Kgm -3, feldspar (an anhydrous aluminosilicate containing K +, Na + and Ca 2+ ions) and small amounts of other minerals such as mica. Small levels of other mineral impurities, such as magnetite, ilmenite (FeTiO 3, which has a structure similar to Al 2 O 3 or Fe 2 O 3 where half the cation sites are occupied by Fe 2+ and half by Ti 4+ ), apatite Ca 5 (OH, F)(PO 4 ) 3 and zircon are also commonly found, the levels of which depend, in part, on the geological area from which the materials in quarried. The colour of the granite is determined by the presence of potassium, calcium, iron and sodium in the feldspar. For this study Granito Asa Branca grade granite powder was used *. This material was chosen due to its low-cost and high volume availability Powder Preparation and Characterisation The powder was initially washed and dried in an oven at 100 C for 12 hours. It was then milled for 20 hours in a ball mill. The powder was classified by sieving using screen apertures ranging from 200 to 80 µm. The powder was characterised by x-ray diffraction, electron microscopy [Philips XL30], and BET analysis Sample preparation and Sintering Pressed powder samples were prepared using a compression mould with a rectangular cavity of dimensions 10 x 20 x 20 mm. 20% weight distilled water was added as a binder. The as-pressed samples were dried at 100 C for 12 hours. Sintering was performed in a muffle furnace for 1 hour at * Imarf, Sobral, Brasil CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10404

5 1000, 1050 and 1100 C respectively using a heating rate of 5 min -1. Cooling to the ambient temperature was allowed to occur naturally. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10405

6 3. Results 3.1 Surface Area The surface area (S) as given by the BET method was 1.69 m 2 g -1. Figure 1 shows a SEM micrograph of the powder after sieving and milling. The particles are not angular but smooth with a large particle size distribution X Ray Diffraction Figure 2 shows the result of the X ray diffraction studies. The phases present are albite (NaAlSi 3 O 8 ), quartz (SiO 2 ) and microcline (KalSi 3 O 8 ). Comparison with the petrographical analysis of the original material is shown in Table 2. The small discrepancies in these results is probably due to experimental error associated with the different techniques and do not constitute a significant error. 3.3 Optimisation of Organic Vehicle The organic vehicle used greatly affects the part being moulded during subsequent processing. Viscosity measurements showed that the optimum granite powder content for injection using the lowpressure injection moulding system was 82% weight (approximately 62% volume) at the injection temperature of 140 C. This provided a high solids loading with a low viscosity, as seen in Figure 3. Binder removal, which is often the critical step in ceramic injection moulding was empirically optimised and took 96 hours to completion. Examination of the components after de-biding showed that the process had not introduced any macro defects. Debinding time must be reduced if this process is to be used on a commercial scale. This may potentially be achieved using supercritical carbon dioxide and is currently under investigation. 3.4 Sintering As low quartz is stable up to approximately 900 C initial sintering experiments were performed at 1000 C using pressed samples. The intention of this initial work was not to fully optimise the processing conditions and characterise the components, but rather to establish the viability of injection moulding and sintering granite powder into useable components. Initial investigations into the sinterability of the components were performed on the as-pressed samples. Heating at 1000 C for one hour did not produce densification of the sample and gave a compact of powdery consistency that had CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10406

7 insufficient strength for handling. Electron microscopy showed that the microstructure was similar to the as-sieved and milled powder indicating that insufficient temperature or time had been allowed for sintering to occur. Heating to 1050 C for 1 hour produced a component which could be handled after sintering and the microstructure as shown by scanning electron microscopy (Figure 4) was shown to be reasonably dense. Almost complete density (Figure 5) was obtained by heating to 1100 C for 1 hour. This produced an extremely hard component. Shrinkage was shown to be in the region of 10%, but this did not result in the generation of cracks or other macro distortions. After demonstrating the viability of sintering the granite into a dense, homogeneous microstructure injection moulded samples were sintered using the optimised conditions. In this case the shrinkage was shown to be 8% and a fully dense microstructure (Figure 6) was produced. Further work will report the characterisation and properties of the components produced. 4. Conclusions This paper presents the initial results of work into one potential use of waste granite powders. It clearly demonstrates the technical viability of this material for low-pressure injection moulding of simple components. The possibility of manufacturing simple components, such as thread guides, in this manner is certainly an attractive proposition in terms of both reduced environmental impact and reduced production costs. Further work, currently in progress, is needed to characterise the properties of these components and to realise their properties fully. However, it is clearly seen that the manufacture of non-load bearing components from waste powders is a distinct possibility. 5. Acknowledgements The authors are indebted to CNPq for financial support. The Division of Mineral Technology of NUTEC-CE provided considerable assistance. Materials were donated by OPP Poliolefinas, Piera Lira S.A and IMARF. CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10407

8 6. References (1) Bonamico, Estratégias para a consolidação do setor no mercado externo. Revista rochas de qualidade edição 110. (2) Castelo Branco, S.M.A.,1998, Estudo do Resíduo de Rochas Ornamentais para Fins Tecnológicos, Projeto de Pesquisa para o CNPq. (3) Lima Filho, V. X., 2000, Estudo da viabilidade técnica da substituição dos pós cerâmicos convencionais por pó de granito na injeção de peças cerâmicas à baixa pressão, Congresso Nacional de Engenharia Mecânica, Natal, RN. (4) Padilha, A F., 1990 Materiais, ciências dos materiais, engenharia de materiais.revista metalurgia- ABM nº 391 (5) Gomes, U.U., 1993 Tecnologia dos pós, Fundamentos e aplicções. Editora da UFRN (6) Relatório do NITES, Estudo geológico sobre mármores e granitos., Rochas de Qualidade, Edição 109, pp de CHARACTERISTICS ADVANCED CERAMICS TRADITIONALCERAMICS Raw materials Synthetic Natural Particle size distribution Narrow (d<µm) Wide (100µm<d<1000µm) Processing Strictly controlled Conventional Sintering Low-content or absence of High content of vitreous phase vitreous phase Relative cost 100 to Table 1: A Simple Comparison of Traditional and Advanced Ceramics CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10408

$Figure 1: The Granite Powder after sieving and milling Figure 2: Results Obtained from the X-Ray diffraction of the Asa Branca Powder Albita Quartzo Microclina Minerais não essenciais X Ray 31 %$

9 Figure 1: The Granite Powder after sieving and milling Figure 2: Results Obtained from the X-Ray diffraction of the Asa Branca Powder Albita Quartzo Microclina Minerais não essenciais X Ray 31 % 58 % 11 % _ Petrographic Analysis 33 % 49 % 10 % 8 % Table 2: Composition of Asa Branca Granite CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10409

sintered at 1100C for 1 hr CONGRESSO BRASILEIRO DE

10 Figure 3: Viscosity of the Organic Vehicles Figure 4: Sample sintered at 1050C for 1 hr Figure 5: Sample sintered at 1100C for 1 hr CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10410

11 Figure 6: Injected samples sintered at 1100C for 1 hr CONGRESSO BRASILEIRO DE ENGENHARIA E CIÊNCIA DOS MATERIAIS, 14., 2000, São Pedro - SP. Anais 10411

PORCELAIN TILES BY THE DRY ROUTE

PORCELAIN TILES BY THE DRY ROUTE (1,) F. G. Melchiades, (1) M. T. Daros, (1,) A. O. Boschi (1) Laboratório de Revestimentos Cerâmicos - LaRC Departamento de Engenharia de Materiais - DEMa Universidade