Effect of Kaolin, Quartz, and Flux Content on the Porosity and Thermal Expansion Coefficient of Triaxial Bodies at Low Temperatures

Abstract

Triaxial bodies are well-known in porcelain industries. Using kaolinite, quartz, and feldspar as main constituents of raw material has many advantages, and makes calculations for body recipe much easier. Literature covers information for temperatures as high as 1400°C, which is higher than the soaking temperatures normally used in the porcelain industry. In this study, a pattern is developed for triaxial bodies at 1230°C. Comparison of this pattern and the pattern for 1400°C in literature shows that the production of dense bodies would be more restricted at a lower temperature. The glassy area at the proposed pattern is smaller, whereas porous and brittle areas at the pattern of 1230°C are wider than these areas at the pattern for higher temperatures. A linear relationship is established between flux content and porosity of triaxial bodies.

Keywords

Porcelain; Flux; Triaxial bodies Thermal expansion; Industries.

Introduction

Formulating body composition is one of the main problems in porcelain industries [1]. The process of formulating body composition requires good knowledge on ceramic science and the relationship among the raw materials, processing, and properties of the fired products, as well as satisfies the needs of the production line [2]. For a new plant, the formulation of other factories cannot be copied even with the same technology [3]. However, numerous routes can be used to finalize a body formulation for a given production line. One of the least functional methods is based on the oxide contents of mixtures; its main disadvantage comes from the fact that oxides have different tasks at different phases [4]. For instance, SiO₂ decreases the thermal expansion coefficient in the glassy phase, but increases it in quartz or cristoballite form. Therefore, predicting the final properties of porcelain by the addition of a given oxide is not always possible [5]. Body calculation according to Kaolin, Quartz, and Flux (KQF) contents has many advantages over oxide-based calculations:

• Raw materials used in porcelain industries can be simplified and placed under three categories. Mineral constituents of raw materials can be considered as plastics, fillers, or fluxes [6].
• Each category of raw materials has a predictable effect on the most important properties of a body at either fired or unfired state. Figure 1 shows the effect of kaolin, quartz, and feldspar addition on the most important properties of porcelain bodies [7].

 

Figure 1. The effect of clay, quartz or feldspar addition on most important properties of a recipe.

When formulating a porcelain body, thermal expansion must be monitored, and SiO₂ as quartz need to be increased to form the necessary compounds [8]. Considering SiO₂ as a chemical constituent irrespective of its crystalline form is incorrect. Although this is not a unique method to change thermal expansion but, however, when we are about to changing recipe there is no better solution [9].

The KQF content of the same porcelain product produced in different plants is similar (Figure 2).

 

Figure 2. Ceramic raw materials and early triaxial bodies. A) Semi porcelain; B) Hard porcelain; C) Soft porcelain; D) Chinese porcelain; E) Japanese porcelain; F) Raw kaolin; G) Stoneware; H) Stoneware clay; I) Sands; J) Feldspars; K) Feldspar sands; L) Washed kaolin.

Thus, calculating the recipe for a given porcelain type would be very easy. Referring to Figure 2 and having available raw materials which their chemical analysis is simplified to plastics, fillers and fluxes, the percentage of raw materials in a way that they provide same KQF content is calculated. However, this method is still under debate because of a wide variety of raw materials used in this industry, and the fact that firing temperatures used are not equal. Figure 3 shows the situation of samples containing different amounts of kaolin, feldspar, and silica or quartz, which are fired at SK 9/10.

 

Figure 3. Sintering behaviour of materials in the system kaolin-feldspar-quartz when fired at SK 9/10. P) Porous; D) Compacted, porcelain like; E) Softened, deformed; G) Molten; M) Brittle.

However, this procedure cannot be used to design a porcelain body at lower temperatures. The use of the lowest temperature is due to various reasons, but lower cost of firing is the most important one. Firing porcelain in temperatures as low as 1230°C to 1280°C noticeably reduces cost, and has little effect on the properties of the finished ware. Lowering temperature reduces the costs of kiln furniture, kiln structure, and consumed fuel. The use of lower temperature also has environmental benefits; these considerations explain why the firing temperature of most porcelain industries (e.g., Iran) has been reduced to less than 1320°C.

A diagram that represents low temperature (Figure 3) may be useful in formulating porcelain bodies. However, the use of the KQF method is not the entire process of designing a recipe for a porcelain producing factory, but is merely the first step. The KQF method should be followed by calculations of available raw materials and a few experiments on the proposed recipes. The recipes can be adjusted according to the results following the chart shown in Figure 1 or personal experience. Even after passing a pilot plant production, the final recipe should be monitored during mass production. Figure 4 presents a chart that can be used to finalize a recipe.

 

Figure 4. Flow chart for designing a recipe using KQF method.

The use of such diagrams as Figure 3 to demonstrate the physical properties of porcelains and stoneware’s are common. For instance Bernardin et al., reported the pyroplastic properties of triaxial bodies with different coordinates. Lassinantti Gualtieri et al., showed the physical properties of samples fired at 1220°C for 5 min at triaxial representation, and used feldspar, kaolin, and kaolinitic-Illitic clay as coordinates with quartz sand content kept constant. The present study highlights the effect of kaolin, quartz, and flux contents on the porosity and thermal expansion coefficient of triaxial bodies at low temperatures.

Materials And Methods

Experimental procedures

Table 1 shows the oxide analysis and the calculated mineralogical estimation of raw materials. Zettlitz kaolin from Sedlecky, quartz of Hamedan, Iran, and feldspar SF11 from Setabran Co., Iran were used to provide the required minerals. Particle size distribution analysis was performed using PSA Cilas 920 (Figure 5).

Table 1. Oxide and calculated mineralogical analysis of raw materials.

 

Figure 5. Particle size distribution of raw materials.

A total of 66 samples were made using different kaolin, quartz, and feldspar contents. Raw materials were weighed (± 0.01 g) and mixed in a slip for 6 h. The slips were deflocculated overnight and cast in plaster molds. When deflocculating all samples was no longer possible due to the lack of clay minerals, the samples were cast at the highest possible pint weight. After drying and achieving a constant weight, samples were fired up to 1230°C in an electrical furnace. Figure 6 shows the firing curve of the samples.

 

Figure 6. Firing curve of samples.

Given that it is not possible to have a sample with 100% clay content using the present raw materials, an approximation between the point of 100% kaolin and pure Zettlitz was used to verify the state of 100% clay at 1230°C. The situation is the same for the point of 100% flux. After firing, the samples were inspected by their appearance, and their porosity was measured according to BS 1902. Thermal expansion coefficient of the samples was measured by Netzsch 402E, and the polished and etched sections of the bulk materials were examined using a S360 Cambridge 1990 scanning electron microscope. The samples were etched in 2% HF solution for 5 min.

Results And Discussion

Figure 7 shows the porosities of the samples as vol%. The addition of quartz to clay increases porosity incessantly, whereas the addition of feldspar to kaolin or quartz sharply reduces it.

 

Figure 7. Porosities of samples containing various KQF contents.

This phenomenon can be attributed to the 6% feldspar, which is in kaolin, or to a better compaction as a result of the different size distributions of kaolin and quartz. The addition of feldspar to the clay and quartz mixtures resulted in sharp reduction of porosity (Figures 8 and 9).

 

Figure 8. Porosity vs. flux content in weight % for mixtures at constant clay content.

 

Figure 9. Porosity vs. flux content in weight % for mixtures at constant quartz content.

However, the addition of quartz to clay at constant flux content had no effect on the porosity of the body (Figure 10).

 

Figure 10. Porosity vs. quartz content in weight % for mixtures at constant flux content.

The map of state of the samples and their appearance shows logical results.

Figure 11 illustrates the result of the appearance assessment of the samples fired at 1230°C.

 

Figure 11. Results of appearance assessments of samples fired at 1230°C. P) Porous; D) Compacted, porcelain like; E) Softened, deformed; G) Molten; M) Brittle.

Comparing Figures 3 and 11, the following results can be obtained:

• The area of the porcelain-like samples is more restricted.
• The area of brittle samples is vaster.
• The areas of the fused and glassy samples are smaller than that of the porcelain-like samples.

Figure 12 shows the linear part of Figure 8, and presents a reasonable linear relation between flux content and porosity of porcelain. Approximately 1% increase in flux content reduces the porosity to 1%. Dilatometric investigation of some samples reveals that firing temperature affects the thermal expansion coefficient of porcelain. Table 2 shows the mineralogical content of the samples and their thermal expansion coefficient.

 

Figure 12. Linear relation of porosity and flux content in weight%.

Table 2. Mineralogical content and thermal expansion coefficient of 3 selected recipe.

The addition of quartz expectedly increased the thermal expansion coefficient of the samples at 1230°C. The glass phase and its thermal expansion coefficient have vital effects on the thermal expansion coefficient of porcelain. Quartz grains are dissolved by the surrounding glass phase. At 1230°C, the glass phase around the quartz grains is less than that at 1400°C (Figures 13 and 14).

 

Figure 13. SEM images of sample code 442 fired at 1230°C. Q) Quartz; PM) Primary mullite; G) glass phase.

 

Figure 14. SEM images of sample code 442 fired at 1400°C. Q) Quartz; PM) Primary mullite; G) Glass phase; SM) Secondary mullite.

The glass phase is rich in SiO₂, and its thermal expansion coefficient would be much lower than the glass phase formed at the early stages. Hence, cracks are observed around quartz grains at 1230°C and 1400°C. At higher temperatures, wherein diffusion rates are higher, more glass phases are seen around quartz grain, and the thermal expansion coefficient is lower. Hence, the thermal expansion coefficient of the glass phase at samples with different quartz amounts reaches the same number regardless of the quantity of dissolution centers (quartz amount in bodies), which explains why the thermal expansion coefficient of samples fired at 1400°C with different quartz amounts does not differ too much.

Conclusion

The KQF method can be used to attain an ideal recipe for porcelain production even in temperatures as low as 1230°C. The formulation of the porcelain body would be more difficult with decreasing temperature, and the samples become more porous. The composition ranges that results in a dense body is restricted at temperatures as low as 1230°C, which seems adequate to mature a dense body.

A good linear relationship between porosity and flux content of body is established. A linear relationship between the porosity of fired samples and flux content at 1230°C was established, but this relationship exists only when either the quartz or clay content of the recipe is kept constant. To reduce the thermal expansion coefficient of porcelain at a higher temperature, reducing the quartz content of the body has less effect than reducing the firing temperature. Therefore, the quartz content of the recipe has greater effect on the thermal expansion coefficient of porcelain at lower firing temperatures, indicating that the thermal expansion coefficient and amount of glass phase controls the thermal expansion of porcelain.

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