- Open Access
To blend is a good solution to improve the heat insulation applicability of diatomite ores
© The Author(s) 2018
- Received: 11 October 2018
- Accepted: 6 November 2018
- Published: 27 November 2018
The idea of mixing different grades of diatomite ores with certain ratios to obtain blends with tailored minerals constitution that can help on improving their thermal insulation applicability is introduced in this study. In this respect, three diatomite samples were collected for investigation from El-Masakheet, Kasr El-Sagha, and Demia deposits, El-Fayoum, Egypt. Complete chemical and phase analyses of these samples were carried out. The analyses showed that El-Masakheet sample was of low-grade diatomite with excess calcite content. In addition, Kasr El-Sagha and Demia samples were of high-grade diatomite with minor calcite contents. By firing the dried Kasr El Sagha and Demia yellow samples specimens at different temperatures for 1 h, they showed complete destruction at 900 °C. Different amounts of El-Masakheet sample was blended with Kasr El-Sagha and Demia yellow samples. The specimens were subjected to firing at different temperatures for 1 h. The physical properties of the fired mixed specimens were followed up.
Results showed that by blending El-Masakheet sample with 50% mass ratio with Kasr El-Sagha or Demia yellow samples, a pronounced increase in the thermal applicability of the fired blends from 900 to 1100 °C was achieved. In addition, a notable increase in their open porosity was recorded. This improvement was attributed to the change in the minerals constitution ratios in the blends due to the increase in their calcite contents after El Masakheet addition, especially the Ca/Si and Ca/(Al + Si) atom ratios. This change served in forming high melting points minerals like α-wollastonite and akermanite on firing the blends specimens instead of low melting point minerals like cristobalite and alkali aluminosilicate which were formed on firing the high-grade ores alone. Consequently, an increasing in the porosity and the thermal applicability of the new specimens from 900 to 1100 °C was achieved.
The idea of mixing different grades of diatomite ores with certain ratios to obtain blends with tailored minerals constitution can help on improving their thermal insulation applicability and their open porosity. Besides, this concept gave a good solution to get use of the unexploited low-grade diatomite ores, especially of high calcite content.
- Diatomite ores
- Heat treatment
- Heat insulation applicability
Insulation or thermal insulation is the process of reducing or increasing heat transfer between two or more objects that are in the range of thermal contact or relative influence. Materials that help in reducing heat gain or loss by offering a barrier within the areas are known as insulation materials. The insulation capability of the materials depends on their thermal conductivity. Materials that provide superior insulation especially in industrial applications are known as high-performance insulation materials. Some of the key attributes of high-performance insulation materials are superior heat regulation without having a support of air conditioning machine or heater, prevention from moisture damage, and an optimum in-house atmosphere. High-performance insulation materials also prevent fire damage. As per claims made at the Chalmers Energy Conference 2013, high-performance insulation materials can help reduce energy usage across sectors by 50% by 2050 (Lin and Wang 2009; Qian and Li 2018; Sari et al. 2018; Yao et al, 2018).
Growing awareness regarding greenhouse emissions and rising demand for energy savings are major factors boosting growth of the global high-performance insulation materials market. High-performance insulation processes are in high demand in the industrial sector, mainly due to its natural ability to regulate temperature. This is a very important attribute, as this helps avoid costs related to artificial thermal regulators such as air conditioners and heaters. Energy usage during insulation processes is minimal, which helps to cut down on associated production costs. However, some of the disadvantages associated with the usage of high-performance insulation processes are high set up and maintenance costs and relatively low service life. High-performance insulation materials need periodic maintenance, as majority of the materials are made of aerogel and ceramic fiber (Yurkov 2005; Skalkin 2006).
Asia Pacific is projected to be the fastest growing region in the global high-performance insulation materials market, mainly due to rampant growth of end-use industries such as construction and automotive, especially in emerging economies of India and China. As per stats released by Indian Brand Equity Foundation—part of the Department of Commerce, Ministry of Commerce and Industry, Government of India—the real estate market is expected to be valued at US$ 180 billion by 2020. The automotive sector in India has attracted FDI worth US$ 15.79 billion during April 2000–September 2016, as per statistics released by the Department of Industrial Policy and Promotion (DIPP). On the other hand, Europe is expected to hold the largest share of the market throughout the forecast period owing its large base of end use industries and high awareness about greenhouse effect and lower energy usage among people.
Insulating diatomite bricks are often prepared by firing the ore at 1000 °C through one of three methods: semi dry, plastic, and foam slip methods. To increase the porosity feature of the diatomite bricks, sawdust or cokes are sometimes used as pore former agent. Bricks made of diatomite are characterized by low thermal conductivity, high strength, and chemical resistance. Therefore, diatomite brick is used in ferrous and nonferrous metallurgy, power engineering, and glass application, in addition of cement and petrochemicals industries (Kashcheev et al. 2009a; Tsibin 1966; Kazan Musuem 1976; Ivanov & Belyakov 2008; Kashcheev et al. 2009a; Hanna et al. 2012; Ibrahim 2012; Da et al. 2014; Lui et al. 2017; Sari et al. 2018).
When diatomite is heated to above 1000 °C, its amorphous silica structure is damaged and converted to cristobalite (Kashcheev et al. 2009b; Ibrahim and Selim 2012). The phase transformation of cristobalite may be the reason of the strength reduction of the diatomite bricks. In addition, the possible reactions between the accompanied accessory minerals in the crude diatomite ore with the active diatomitic silica, on firing to above 1000 °C, may form different amounts of liquid phases, which cause deformation in the diatomite bricks. In order to overcome these problems and to increase the applicability temperature of diatomite as well, calcium oxide was added, in the form of chalk, with different mass ratios from 0 to 30%. This reaction between chalk and the diatomitic silica will result in the formation of wollastonite (CaO.SiO2) mineral and accordingly, the chance of cristobalite formation will be decreased, resulting in increasing the applicable firing temperature (Kashcheev et al. 2009a; Meradi et al. 2015; Merado et al. 2016; Galzerano et al. 2018).
In Egypt, diatomite ores are found in three main areas, El-Fayoum Depression (Western Desert), Ras Ghareb (Suez Gulf-Red Sea), and some scattered deposits in the west of the Northern Coast of Egypt (Ibrahim and Powers 2009). The diatomite deposits that are recorded at El-Fayoum Depression are in the north and south west of the Governorate. In the north, they are found in the north and northeast of Lake Qarun, in three areas, Kom Osheem in the East, Qasr El Sagha, and Demia (yellow and gray) in the West. In addition, diatomite ores cover a large area in the southwest of El-Fayoum between Wadi El Rayan and the small villages in the SW of El-Fayoum, particularly to the southeast of Gabel Elow El-Masakheet. This area is located to the SW of the EL Fayoum city by about 28 km and is called El-Masakheet. These deposits were formed in fresh water lakes that was covered most of El-Fayoum Depression in the Halocene era; lakes Moeris (Abdel 1958; Basta et al. 1971; Basta et al. 1972; Faris and Girgis 1969). The main minerals constitutions of these deposits are diatomite (amorphous silica), silica sand, inorganic carbonate (calcite and dolomite), clayey minerals, and organic materials (Ibrahim and Selim 2012). This means that the Egyptian diatomite contains beside silica, different oxide contents of calcium, magnesium, aluminum, and iron (Loukina et al. 1994; Hassan et al. 1999).
In connection with the sharp increase in the price of heat carriers, the use of materials that are made from diatomite as effective heat insulation is becoming more attractive (Hamdi and Hamdi 2013; Hanna et al. 2014; Hanna et al. 2015; Loganina and Karpova 2015; Öztaş et al. 2016; Da et al. 2014; Han et al. 2017). The drawback of heat-insulation materials made of diatomite is their low temperature of prolonged service in heating equipment (ordinarily no higher than 950 °C). Consequently, work on increasing the temperature level for prolonged service of heat insulation is a big challenge.
Three diatomite samples were collected from El-Fayoum depression, namely Masakheet (M2), Demia yellow (D2), and Kasr El Sagha (K2). The chemical composition of the samples was determined using Brunauer-Emmett-Teller and X-ray fluorescence (XRF), while their mineral composition was identified using a Philips X-ray diffractometer Model PW/1710 Cu K α radiation with Ni filter at 40 kV and 30 mA. The scanning speed was 2Ɵ = 2°/min. Particle size analysis of the samples was measured using BT-2001 laser particle size analyzer. The surface area and the porosity measures were determined using Brunauer Emmett-Teller unit. The bulk density and open porosity of the specimens were determined by water displacement after boiling for 2 h to eliminate residual air bubbles. A Vernier caliber with a precision of 0.005 cm was employed to measure the change in dimensions of the tested specimens.
Blended specimens preparation
Different amounts of 5, 15, 30, 40, and 50% by mass of El-Masakheet (M2) sample was blended with Kasr El-Sagha (K2) and Demia yellow (D2) samples. During mixing, appropriate amount of water was added. The resulted mixed masses were molded under low pressure into small cubic specimens of two inches length. The specimens were left to dry in two different conditions: at ambient temperature for 48 h at 110 °C for 24 h. The dried specimens were then subjected to firing at 1100 °C for 1 h. The physical properties of the fired mixed specimens such as the change in dimension, bulk density, and open porosity were followed up. The phase analyses of some fired specimens were followed up.
Characterization of the original raw diatomite samples
Chemical composition of raw diatomite samples before and after firing
Original samples, %
After firing, %
Ca/Al + Si
Al/Al + Si
Ca/Si + Mg
After firing at 1000 °C for 1 h., the SiO2 content in the three samples increases to 24.36%, 70.86%, and 67.51%, respectively (Table 1). The CaO content increases to 58.59%, 3.38%, and 8.31%, respectively (Table 1). The alumina and magnesia content increase to 4.89 and 2.16% in M2, 11.26 and 1.21 in K2, and 11.20 and 1.75% in D2 (Table 1).
The ratios Ca/Si and Ca/Al + Si reach 2.58% and 2.08% in the fired sample M2, where they reach 0.05%, 0.04% in the fired sample K2 and 0.12%, 0.10% in the fired sample D2 (Table 1). In addition, the ratios Al/Al + Si and Ca/Si + Mg reach 0.19% and 2.27% in the fired sample M2, where they reach 0.11%, 0.05% in the fired sample K2, and in the fired sample D2 these ratios reach 0.15% and 0.13% (Table 1).
Surface area and pore volume measures of the samples
Kasr El Sagha 2
Demia yellow 2
Surface area m2/g
Pore volume cc/g
Characterization of the fired raw diatomite samples
The Al2O3/SiO2 mass ratio of M2, K2, and D2 samples are 20.09, 15.90, and 16.59%, respectively. Therefore, the low Al2O3/SiO2 mass ratio of K2 and D2 samples may be the reason of their lower softening point. The presence of alkalies even in small amount with the low Al2O3/SiO2 % renders the formed liquid phase after firing, more fluid (Hinz 1972). On firing the K2 and D2 samples, the present alkali feldspar transformed to Na and K tecto alumino-silicate leading to a decrease in the softening point of the fired samples (> 900 °C). In addition, the high CaO content and consequently the high CaO/SiO2 mass ratio in M2 sample leads to formation of dicalcium silicate and calcium magnesium alumino-silicate which increase the softening point of the fired M2 sample. This clearly noticed where Ca/Si, Ca/Al + Si, and Ca/Si + Mg in sample M2 are higher than Al/Al + Si atom ratio (Table 1).
Characterization of the M2-K2 and M2-D2 fired blends
M2-K2 fired blends
Chemical composition of fired M2-K2 specimens after firing at 1100 °C for 1 h
M2 addition by mass%
Ca/Al + Si
Al/Al + Si
Ca/Si + Mg
M2-D2 fired blends
Chemical composition of different mixtures of M2-D2 diatomite specimens after firing at 1100 °C for 1 h
M2 addition by mass%
Ca/Al + Si
Al/Al + Si
Ca/Si + Mg
It is remarked that there is complete absence of the anorthoclase phase in the XRD patterns of M2-K2 and M2-D2 specimens at (50/50% mass) (Fig. 13). In addition, it is noticed that when M2 sample is added to K2 or D2 samples and according to the increase in the calcium content in the new blends, the values of Ca/Si, Ca/Al + Si, and Ca/Si + MgO atomic mass ratios increase from 0.05, 0.04, and 0.05 in K2, and 0.12, 0.10, and 0.13 in D2 to 0.58, 0.49, and 0.55 in M2-K2 and 0.66, 0.57, and 0.64 in M2-D2 (Tables 1, 3, and 4). The increases in the values of these ratios lead to the formation of wollastonite and akermanite in the fired blends at the expense of cristobalite and alkali alumino-silicate.
However, the presence of alkalis even in small amount beside low alumina/silica weight ratio renders the liquid more fluid. The alkalis that are present in the two samples as alkali feldspar and forms on firing Na and K tecto alumino-silicate leading to a decrease of their softening point (> 900 °C). On the other hand, the existence of high CaO content or in other words high CaO/SiO2 mass ratio in El-Masakheet (M2) sample leads to the formation of dicalcium silicate and calcium magnesium alumino-silicate which increases the softening point of El Masakheet (M2) sample (Table 1). This appears clearly from its chemical composition, where Ca/Si, Ca/Al + Si, and Ca/Si + Mg are higher than Al/Al + Si atom ratio, recording 2.58, 2.08, 2.27, and 0.19, respectively (Table 4).
This is in contrast to what was obtained in the case of Demia yellow (D2) and Kasr El sagha (K2) diatomite samples where the values of Al/Al + Si atom ratios (0.15 and 0.11) are higher than the values of Ca/Si, Ca/Al + Si, and Ca/Si + Mg (0.12, 0.10, 0.13 and 0.05, 0.04, 0.05), in the two samples respectively (Table 4).
When El-Masakheet M2 diatomite sample was added to Demia yellow D2 or Kasr El Sagha K2 diatomite samples, the values of Ca/Si, Ca/Al + Si, and Ca/Si + MgO atom ratio increase and at the same time the value of Al/Al + Si atom ratio decreases. This leads to the formation of wollastonite and akermanite at the expense of alkali alumino-silicate. The formed phases have higher melting points reaching 1540 and 1450 °C, respectively. Therefore, the specimens prepared from mixtures show improvement in their thermal stability from 900 to 1100 °C.
Despite high-grade diatomite ores contain high content of opal silica, yet they sometimes show limitation in their thermal applicability as heat-insulating materials. This is due to their low softening points, which depend upon their minerals constitution. In order to circumvent this issue and, in addition, to take advantage of low-grade diatomite ores which have low silica content and in most cases high calcite content, the idea to blend such high grade diatomite with those ores having high calcite content is introduced.
In this study, low-grade El-Masakheet diatomite of high calcite content is blended at 50/50 mass ratio with high-grade Kasr El-Sagha or Demia yellow diatomite ores of low calcite contents. This mixing leads to changes in the minerals constituents’ ratios, especially the Ca/Si and Ca/(Al + Si) atom ratios of the new specimens. On firing, it serves in forming high melting points minerals like α-wollastonite and akermanite instead of low melting point minerals like cristobalite and alkali aluminosilicate, which are formed on firing the high grade ores alone. Consequently, an increasing in the porosity and the thermal applicability of the new specimen blends from 900 to 1100 °C is achieved.
The authors of this research work thank Dr. Khaled Ezzat (CMRDI) for his kind help.
This study was not funded by any source.
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All authors contributed in the practical work of this research. They also share in preparing the manuscript for publication. They approved the final manuscript.
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