Valorization of rice straw waste: production of porcelain tiles

(Valorização de resíduos de palha de arroz: produção de porcelanatos)

Abstract

The rice industry generates huge amounts of rice straw ashes (RSA). This paper presents the results of an experimental research work about the incorporation of RSA waste as a new alternative raw material for production of porcelain tiles. The RSA replaces, partially or completely, the non-plastic raw materials (quartz (feldspathic sand in this research) and feldspar), that together with the clays, constitute the major constituents of formulations of porcelain tiles. A standard industrial composition (0% RSA) and two more compositions in which feldspar and feldspathic sand were replaced with two percentages of RSA (12.5% RSA and 60% RSA) were formulated, keeping the clay content constant. The mixtures were processed, reproducing industrial porcelain tile manufacturing conditions by the dry route and fired at peak temperatures varying from 1140-1260 ºC. The results showed that additions of 12.5% RSA in replacement of feldspar and feldspathic sand allowed producing porcelain tiles that did not display marked changes in processing behaviour, in addition to obtain a microstructure and the typical mineralogical phases of porcelain tile. Thus, an alternative use of an agricultural waste material is proposed, which can be translated into economic and environmental benefits.

Keywords:
porcelain tile; rice straw ash; feldspar; feldspathic sand

Resumo

A indústria do arroz gera enormes quantidades de cinzas de palha de arroz (RSA). Este artigo apresenta os resultados de um trabalho de pesquisa experimental sobre a incorporação de resíduos RSA como uma nova matéria-prima alternativa para a produção de porcelanato. A RSA substitui, total ou parcialmente, as matérias-primas não-plásticas (quartzo (areia feldspática nesta pesquisa) e feldspato), que, juntamente com as argilas, constituem os principais componentes de formulações de porcelanato. A composição padrão industrial (0% RSA) e mais duas composições em que feldspato e areia feldspática foram substituídos por duas porcentagens de RSA (12,5% e 60% RSA RSA) foram formuladas, mantendo a argila constante. As misturas foram processadas, reproduzindo porcelanato nas condições de produção industrial por via seca e queimas em temperaturas máximas variando entre 1140-1260 °C. Os resultados mostraram que adições de 12,5% RSA em substituição da areia feldspato e feldspático permitiram produzir porcelanato que não mostraram mudanças marcantes no comportamento do processamento, além de obter uma microestrutura e as fases mineralógicas típicas de porcelanato. Assim, é proposta uma alternativa de uso de um material de resíduos agrícolas, o que pode ser traduzido em benefícios econômicos e ambientais.

Palavras-chave:
azulejos de porcelana; arroz cinza palha; areia feldspato; feldspática

INTRODUCTION

Porcelain stoneware tile is a ceramic product characterised by low water absorption (≤0.5% according to the standard ISO 13006¹⁾⁾, making it a high-performance material². Porcelain tile is typically used in flooring, wall cladding, and ventilated façades³. In recent years, porcelain tile production and sales have grown, compared withproduction and sales rates of other ceramic construction materials, as a result of its high technological properties, particularly in regard to water absorption and frost resistance, and mechanical properties, such as modulus of ruptureand abrasionresistance^{2), (3) (4), (5}.

A typical porcelain tile composition consists of 40-50% illitic-kaolinitic clay, 10-15% quartz, and 35%-45% feldspar (all percentages by weight)^{6), (7}. Feldspar is a high-cost raw material and replacement would represent a significant reduction in porcelain tile production costs^{8), (9}. Feldspar is a mineralflux commonly used in porcelain tile bodies. However, high-grade feldspathic minerals resources have recently begun to become scarce, thus making it necessary to consider alternative sources of fluxing materials that can form glassy phase at temperatures equal toor lower than those of the feldspars used at present¹⁰. It should be added that deposits of good quality quartz sand in Colombia are limited. As a result, various alternative fluxes (soda-lime glasses, glass scrap (TV/PC cathode ray tubes and screens), blast furnace slag, metallurgical slags, zeolites, rice straw ash (RSA), etc.) and non-plastic materials (rice husk ash(RHA), silica fume (SF), fly ash (FA)) have been incorporated intoporcelain tile body compositions, with a view to studying their effecton product firing behaviour and end-product technical properties^{10), (11), (12), (13), (14), (15), (16), (17), (18), (19), (20), (21), (22), (23), (24}. However, the use of secondary raw materials is considered feasible only if the industrial process remains essentially unaltered and product quality and properties are not impaired^{13), (19}.

As feldspars generally come from just a few regions, e.g. Germany, Turkey, and France, in addition to the limited deposits of good quality quartz sands in Colombia, their possible replacement with rice straw ash (RSA) is an attractive option, taking into account the results of Guzmán et al.²⁴ and the K₂O and SiO₂ contained in RSA, which is of the order of 11.30-12.30% and 74.31-74.67% by weight of the ash, respectively^{25), (26}.

This study was undertaken to examine the feasibility of using rice straw ash (RSA) in ceramic mixtures topartially replacenon-plastic materials (sodium feldspar and feldspathic sand) used in manufacturing porcelain tilebodies.The effects due to the use of RSA were investigated in laboratory experiments and discussed in terms of firing behaviour and physical-mechanical properties.

EXPERIMENTAL PROCEDURE

Materials

The raw materials used in preparing triaxial ceramic bodies were feldspathic sand, sodium feldspar, and illitickaolinitic clay. To obtainthe RSA, amethodology was usedfor the obtainment of RSA by a calcination process of the materialat 800 ºC to eliminate chlorine and sulphuras far as possiblewith a moderate content of potassium in the ash²⁴. After the RSA had been obtained, it was subjected to milling for one hour in a laboratory ball mill. The chemical and mineralogical compositions of the raw materials were determined by X-ray fluorescence and X-ray diffraction, respectively (see Table I). The mineralogical phases are reported with the Inorganic Crystal Structure Database (ICSD) patterns corresponding to the identified minerals.

Table I
Chemical and mineralogical compositions of the raw materials. [Tabela I - Composições químicas e mineralógicas das matérias-primas.]

Element and/or compound	Concentration by weight (%)
	Illitic-kaolinitic clay (A)	Sodium feldspar (F)	Feldspathic sand (AF)	RSA (C)
SiO2	59.00	70.00	91.00	79.62
Al2O3	27.30	18.00	5.00	0.27
Fe2O3	0.93	0.08	0.12	0.26
CaO	0.27	0.50	0.10	2.80
MgO	0.59	0.10	0.01	0.89
Na2O	0.52	9.70	0.10	0.35
K2O	2.45	0.35	2.50	10.53
TiO2	1.45	0.11	0.08	-
MnO	<0.01	-	-	0.71
P2O5	0.05	-	-	1.61
Cl	-	-	-	0.59
S	-	-	-	01.74
Zn	-	-	-	0.01
Rb	-	-	-	0.01
Sr	-	-	-	0.01
Cu	-	-	-	0.01
LOI	7.29	0.50	1.10	0.59
Mineralogical phases	Kaolinite (ICSD 87771)	Albite (ICSD 90142)	Quartz (ICSD 83849)	Cristobalite α (ICSD 74530)
	Muscovite (ICSD 202263)	Muscovite (ICSD 25803)	Kaolinite (ICSD 87771)	Tridymite α (ICSD 1109)
	Quartz (ICSD 90145)	Quartz (ICSD 34636)	Orthoclase (ICSD 159347)
Anatase (ICSD 96946)

The chemical composition, determined by XRF, of the RSA obtained by calcination at 800 ºC showed that the main constituents were SiO₂ and K₂O (see Table I), corroborating the results reported by Jenkins et al.²⁵ and Thy et al.²⁶. With relation to the clay, this consisted mainly of SiO₂ and Al₂O₃. In addition, it displayed low contents of K₂O and TiO₂ (2.5% and 1.5%, respectively), in which the K₂O could act as a flux, while TiO₂ was a chromophore oxide that provided a yellowish hue²⁷. The feldspar, in turn, consisted mainly of SiO₂ and Al₂O₃, as well as 9.7% Na₂O, indicating that it was a sodium feldspar. On the other hand, the feldspathic sand mainly contained SiO₂ and Al₂O₃, in addition to 2.5% K₂O, which could act as a flux.

Ceramics formulation and testing

A standard industrial composition labelled 0% RSA, and two more compositions labelled 12.5% RSA and 60% RSA in which feldspar and feldspathic sand were replaced with two percentages of RSA (12.5 wt.% and 60 wt.%, respectively) were formulated, keeping the clay content constant in all formulations. The chemical compositions of the corresponding porcelain are reported in Table II.

Table II
Chemical composition (wt.%) of porcelain samples as formulated. [Tabela II - Composição química (peso%) das amostras de porcelana, como formulado.]

Element and/or compound	Concentration by weight (%)
	0% RSA	12.55% RSA	60% RSA
SiO2	67.70	67.85	71.37
Al2O3	20.42	18.85	11.08
Fe2O3	0.42	0.44	0.53
CaO	0.37	0.68	1.79
MgO	0.29	0.39	0.77
Na2O	5.07	4.38	0.42
K2O	1.41	2.57	7.30
TiO2	0.64	0.63	0.58
MnO	0.00	0.09	0.43
P2O5	0.02	0.22	0.99
Cl	0.00	0.07	0.35
S	0.00	0.22	1.04
Zn	0.00	0.00	0.01
Rb	0.00	0.00	0.01
Sr	0.00	0.00	0.01
Cu	0.00	0.00	0.01

The replacement of feldspar and feldspathic sand with RSA in the mixture, in general, led to an increase in the SiO₂, K₂O, MgO, CaO, Fe₂O₃ and Cl contents, in addition to a reduction in the Al₂O₃ and Na₂O contents (Table II), the expected behaviour of compositions is, in some extent, unpredictable due to these compositional changes.

In order to obtain the typical particle size of porcelain tile body compositions, one kg of each ceramic formulation wasball-milled to a residue (R) of1.5-2.0% on a 40 μm sieve. Table III details the dry milling times required to prepare the formulations to a residue of 1.5-2.0% on a40 μm sieve. As observed, the higher the RSA content the shorter is the milling time.

Table III
Milling time required to reach a residue of 1.5-2.0% on a 40 μm sieve. [Tabela III - Tempo de moagem necessário para atingir um resíduo de 1,5-2,0% em peneira de 40 μm.]

Mixture	t (min)	% R (40 μm)
0% RSA (10AF;50F;0C;40A)	50	2,2
12.5% RSA (5AF;42.5F;12.5C;40A)	25	2,0
60% RSA (0AF;0F;60C;40A)	17	1,6

In order to determine the behaviour of the ceramic bodies in pressing and firing, cylindrical test pieces about 7 mm thick and 40 mm in diameter, in addition toprism-shaped test pieces measuring 80x20x6 mm,were prepared to determine the mechanical properties. The test pieces were formed at a moisture contentof 5.5 wt.% (on a dry basis) by uniaxial pressing at a pressure of 400 kg/cm². After they had been pressed, the test pieces were dried at 110 ± 5ºC in an electric laboratory oven. They were then sintered in a Pirometrol R electric laboratory kiln with a heating ramp of 70 ºC/min between 25 ºC and 500 ºC, and 25 ºC/min from 500 ºC to the respective peak firing temperature. The residence time at peak firing temperature was 6 min, and the fired test pieces were cooled inside the kiln in order to avoid macroscopic residual stresses. The peak firing temperatures encompassed the range 1140-1260 ºC, at intervals of 20ºC, depending on each composition. The measurements of dried and fired dimensions of the test pieces were made using a digital calliper.

The maximum densification temperature was determined for each mixture from the vitrification curve, constructed after determining the bulk density, using the experimental conditions described previously. Owing to the scarce variation of water absorption with temperature at values below 1%, it was preferable to determine the maximum densification temperature as the characteristic temperature for industrial firing temperatures instead of the temperature at which the water absorption was less than 0.5%.

The technological properties of the fired test pieces were evaluated by performing the following tests: linear shrinkage, apparent porosity, water absorption, and bulk density. The linear shrinkage, LS (%), of fired samples was determined according to the Standard Test Method for Drying and Firing Shrinkages of Ceramic Whiteware Clays²⁸ by means of the following equation:

being L_d and L_f the diameter (mm) of dried and fired samples, respectively.

The bulk densities, BD (g/cm³⁾, ofdried (BD_d) and fired (BD_f) samples were determined by the mercury displacement method measured by means of the following equation:

being m_x the mass of dried (m_d) or fired sample (m_f), ρ_Hgthe density of mercury (13.53 g/cm³⁾ and m_Hg the mass of each sample submerged in mercury²⁹.

The water absorption, WA (%),was measured according to the Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity, and Apparent Specific Gravity of Fired Whiteware Products³⁰, which involves drying the test specimens to constant mass (D), boiling in distilled water for 5 h and soak for an additional 24 h at room temperature. After impregnation, the saturated mass (M) of each specimen is determined. WA expresses the relationship of the mass of water absorbed to the mass of the dry specimen as follows:

The apparent porosity, ε_α (%), expresses the relationship of the volume of open pores to the exterior volume of the specimen and is calculated as follows:

The modulus of rupture was determined analogously to the Standard Test Methods for Flexural Properties of Ceramic Whiteware Materials³¹ by a three-point bending test assembly, the span between supports being 62.2 mm and the load application rate being 5 mm/min. An average of ten measurements was taken for this purpose. In the STD mixture (0% RSA) and the mixture with RSA in replacement of feldspar that exhibited the best modulus of rupture (12.5% RSA), the main crystalline phases were identified by XRD using a PANalytical X'Pert PRO X-ray diffractometer. The microstructural characteristics were observed by scanning electron microscopy, for which the fracture surface of each sample was polished and attacked with a hydrofluoric acid (HF) solution at 5% for 3 min, washed with distilled water and ethyl alcohol, and subsequently dried and coated with carbon.

RESULTS AND DISCUSSION

Feasibility of using RSA as a non-plastic material in porcelain tile compositions

The vitrification curves of the mixtures 0% RSA, 12.5% RSA and 60% RSA are plotted in Fig. 1. Regarding optimum firing range (interval of temperatures where water absorption is lower than 0.5% and bulk density is maximized), it can be observed that 12.5% RSA composition showed a similar temperature range than 0% RSA. Moreover, the 60% RSA composition showed a shorter temperature range than 0% RSA; this indicates that more careful control is required during the firing process of this porcelain. From the vitrification curves, the optimum firing temperatures (those corresponding to maximum density) for each composition were determined to be 1183 ºC, 1183 ºC, and 1234 ºC, respectively.

Figure 1
Linear shrinkage, water absorption and bulk density in the porcelain tile samples (0% RSA, 12.5% RSA and 60% RSA) as a function of firing temperature. 0% RSA appears in all the plots as a standard composition. [Figura 1: Retração linear, absorção de água e densidade aparente das amostras de azulejos de porcelana (0% de RSA, 12,5% e 60% de RSA RSA) como uma função da temperatura de queima. 0% RSA aparece em todas as parcelas, como uma composição padrão.]

The results of the technical properties obtained for each dried as well as fired composition, at its respective optimum firing temperature, are detailed in Table IV.

Table IV
Physical and mechanical properties of the dried and fired test pieces obtained at their optimum firing temperature. [Tabela IV - Propriedades físicas e mecânicas das peças obtidas via seca e tratadas a temperatura ótima de queima.]

Technical properties	0% RSA	12.5% RSA	60% RSA
Dry bulk density (g/cm3)	1.91	1.8	1.65
Optimum firing temperature (ºC)	1.183	1.183	1.234
Linear shrinkage (%)	8.1	8.7	8.6
Fired bulk density (g/cm3)	2.43	2.36	2.12
Water absorption (%)	<0.1	<0.1	0.2
Apparent porosity (%)	<0.1	<0.1	0.5
Modulus of rupture (MPa)	69	67	48

The increase in the quantity of RSA in the mixture decreased the compactness of the test piece, evidenced by a reduction in dry bulk density. This behaviour could stem from the fact that it adversely affected the balance between the non-plasticand the colloidal plastic particles, raising the linear shrinkage of the fired test pieces. Compositions 12.5% RSA and 60% RSA displayed this behaviour, with an increase in linear shrinkage in comparison with standard mixture 0% RSA (8.1%). The optimum vitrification range is achieved when apparent porosity reaches a minimum value, tending to be nearly zero and simultaneously bulk density and linear shrinkage are maxims. Firing above the vitrification range has a drastic adverse effect on the physical properties owing to an increase in the pressure of the gases trapped in the pores, producing swelling or bloating of the piece⁴. The maximum bulk densityof each composition decreased as the replacementof feldspar and feldspathic sandwith RSA increased, in comparison with that of standard mixture 0% RSA, as a result of the lowerdry bulk density of the compositions with RSA. Mixture 12.5% RSA exhibited the highest bulk density (2.36 g/cm³⁾ with respect to the other mixture with RSA. Thus, mixture 60% RSA displayed a reduction in bulk density (2.12 g/cm³⁾ to the extent of being lower than the minimum value of 2.30 g/cm³ specified by the European standard UNI EN 87³².

The apparent porosity and water absorption percentage of mixture 12.5% RSA displayed similar trend to those of standardmixture 0% RSA. However, mixture 60% RSA exhibited higher values, which could be attributed to different factors, such as their low dry bulk density and the bloating caused by the greater content ingas-generating substances (Fe₂O₃, Cl, andLOI) in the RSA, in addition to the increase in the quantity ofSiO₂, which reduced the sintering capacity, and the increase in the quantity of liquid phase of potassium origin, increasing its viscosity in comparison with that of the liquid phase of sodium originin agreement with the findings previously reported^{7), (33}. These phenomena were corroborated by the decrease in shrinkage and increase in porosity displayed by mixture 60% RSA (see Table IV), typical behaviourof the bloating phenomenon during liquidphase sintering of traditional ceramics⁴.

The modulus of ruptureof the fired test pieces (see Table IV) evidences a similar trend to that displayed by fired bulk density. The previous phenomena are consistent with reported findings⁸, who noted that generally, at greater bulk density, the modulus of rupture increased as a consequence of reduction of porosity. According to the criteria of standard IS0 13006 for dry-pressed ceramic tiles, ceramic tile is defined as porcelain stoneware tile belonging to group BIa, when it exhibits a modulus of rupture ≤35 MPa and water absorption ≤0.5%. In view of the results of the modulus of ruptureand water absorption (see Table IV), compositions 0% RSA, 12.5% RSA and 60% RSA could all be deemedas porcelain stoneware tile belonging to group BIa. However, in composition 60% RSA, there must also bean increase in the optimum firing temperature. In addition, the optimum firing range was very narrow (see Fig. 1), reducing the feasibility of using these mixture on an industrial scale.

The SEM micrographs at 100X in backscattered electron mode of the polished surfaces (see Figs. 2a and 2b) showed that the surface of the test piece fired under maximum densification conditions of composition 12.5% RSA displayed a greater quantity of closed pores than the test piece of composition 0% RSA; which was consistent with the density and porosity values noted previously.

Figure 2
SEM micrographs in BSE (backscattered electron) mode of polished surfaces of fired test pieces 0% RSA (a) and 12.5% RSA (b) (100X); and SEM micrographs in SEI (secondary electron imaging) mode of polished surfaces of fired test pieces 0% RSA (c) and 12.5% RSA (d) (10000X). [Figura 2: Micrografias obtidas em mivroscópio eletrônico de varredura em BSE (elétrons retroespalhados) de superfícies polidas das peças queimadas com 0% RSA (a) e 12,5% RSA (b) (100X); e micrografias em SEI (imagem de elétron secundária) de superfícies polidas das peças queimadas com 0% RSA (c) e 12,5% RSA (d) (10000x).]

The SEM micrographs at 10000X in secondary electron imaging (SEI) mode (see Figs. 2c and 2d) showed the presence of primary mullite crystals (M) and quartz crystals (C) embedded in a glassy phase (GP) in the microstructure of fired test pieces of compositions 0% RSA and 12.5% RSA. The SEM results were corroborated by XRD tests of fired test pieces of compositions 0% RSA and 12.5% RSA (see Fig. 3), highlighting the presence of α-quartz (2θ = 20.86º and 26.64º) (ICSD 83849), albite (2θ = 22.03º and 27.91º) (ICSD 240519),and mullite (2θ = 16.43º; 33.21º; 35.26º; 39.24º, and 40.87º) (ICSD 99328), this last phase being partially responsible for porcelain mechanical strength. In addition, in 12.5% RSA the presence of α-cristobalite was evidenced (2θ= 21.76º) (ICSD 74530), basically stemming from the RSA (see Table I) in agreement with reported findings³⁴.

Figure 3
X-ray diffraction patterns: 0% RSA (a) and 12.5% RSA (b) (M = mullite, C = quartz, Cr = cristobalite, A = albite). [Figura 3: Difração de raios X: 0% de RSA (a) e 12,5% de RSA (b) (M = mulita, C = quartzo, Cr = cristobalita, A = albita).]

CONCLUSIONS

Rice straw ashes (RSA) displayed anon-plastic character, allowing them to be used as partial replacement of feldspar and quartz in porcelain tile compositions. No pronounced change took place in the technological process when RSA was added to the composition in a quantity of 12.5%, in replacement of feldspar and feldspathic sand, allowing the obtaining of mineralogical phases and microstructure of typical porcelain tiles.The fired test pieces of all the compositions studied with a RSA addition (12.5% RSA and 60% RSA) exhibited the characteristics required for porcelain stoneware tile, BIa group (modulus of rupture ≥ 35 MPa and water absorption ≤ 0.5%) according to standard ISO 13006. However, in the composition in which RSA completely replaced feldspathic sand and feldspar, 60% RSA, the optimum firing range was narrower, in addition to requiring higher firing temperatures. This reduces the feasibilityof using this last mixtureon an industrial scale.

ACKNOWLEDGEMENTS

The authors would like to thank Universidad del Valle (Colombia), Instituto de Tecnología Cerámica (ITC) (España) and COLCIENCIAS (Colombia) for their support provided to conduct this study. In particular, this paper presents the partial results from the research project "Triaxial ceramics based on rice straw ash", code 110652128358, supported by COLCIENCIAS, Official Call 521 of 2010, contract RC. No. 325-2011.

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Publication Dates

• Publication in this collection
  Oct-Dec 2015

History

• Received
  01 June 2015
• Accepted
  23 July 2015