Use of Red Mud in the Production of Ceramic Foam Materials for Thermal Insulation Applications

Introduction

As global population increases daily, the limited natural resources used to meet the living needs of this population are declining significantly. Energy is one of humanity's most basic needs, and energy demand in both industrial and domestic applications is accelerating due to rising population and advancing technologies. For this reason, topics such as efficient energy use, renewable energy sources, and development of low-energy machinery and systems remain on the agenda. Today, research into efficient use of existing energy sources is equally important as investigation of new and renewable energy sources. In our country, approximately 85% of total energy consumption in buildings is spent on heating and cooling depending on the season. Energy loss resulting from uninsulated or poorly insulated walls can reach very high values of around 50%. This clearly demonstrates how important insulation is. High energy consumption, besides causing significant economic losses, also brings other important problems such as air pollution, increased greenhouse gas emissions, and environmental damage.

Without proper thermal insulation in buildings, excessive energy consumption occurs to provide heating in winter and, conversely, cooling in summer.

To reduce energy used for heating and cooling across all four seasons in buildings to minimum levels, proper insulation must be applied. With efficient thermal insulation material applications in buildings, significant energy savings can be achieved, and this allows for serious contributions to the national economy while also reducing environmental damage through less energy consumption. Materials used in building insulation applications are generally grouped under two basic categories: inorganic and organic. Important advantages of inorganic materials include: Class A non-flammable properties, very low thermal conductivity values (0.030–0.045 W/m.K), the ability to be used over a wide temperature range (between −100–750°C), and higher resistance to moisture compared to organic materials. Among inorganic materials, ceramic-based materials commonly used as foam or fiber-wool in the form of glass or stone are employed. Looking at the European market, the share of materials in this group is 60%, while organic materials such as polystyrene and polyurethane account for 27%. The most important problem encountered in fiber inorganic material use is the carcinogenic effect of fine fiber materials. Therefore, inorganic fibers present a potential health risk during application and use. This risk is eliminated through the use of foamed inorganic materials. Examples of inorganic ceramic foam materials used include calcium silicate, expanded clay aggregate, perlite, and vermiculite. In the last decade, closed-cell panels, vacuum insulation panels, gas-filled panels, aerogels, and phase-change materials have emerged as innovative applications in the insulation sector. Closed-cell panels provide a 40% thickness reduction compared to glass wool while showing no difference in thermal insulation performance. With increasing population and developing industries, large quantities of solid domestic and industrial waste have emerged as a significant problem today. This situation prevents efficient use of our planet's limited natural resources and causes serious environmental problems. Therefore, under current conditions, converting solid waste into new products has emerged as an important need.

What is Red Mud?

Red mud is an industrial solid waste with a complex structure generated during alumina production through the Bayer process. During the leaching process of bauxite mineral used in alumina production with caustic soda, typically 1–1.5 tons of red mud are generated per ton of alumina produced. [1] Alumina is used in many different industrial applications as well as in the production of aluminum metal. Approximately two tons of alumina are used for the production of one ton of aluminum metal. Aluminum, which has the most widespread use in the non-ferrous metal industry, has very large production volumes worldwide. Storage of the large quantities of red mud generated for aluminum and alumina production facilities presents a very significant problem.

[2] Large quantities of red mud are generated as waste during alumina and aluminum production, but only approximately 10% of this waste is used in different application areas. [3]

Intensive scientific research is ongoing on the use of red mud in different application areas including production of cement [4-6], concrete [7-9], geopolymer [10], ceramic [11-15] products, recovery of valuable metals (Fe, Ni, Ti and Si) [16-18], removal of organic pollutants [19], dyes [20, 21], lead [22], cadmium, copper, arsenic and other toxic pollutants [23-26] from water. Numerous scientific research studies are also being conducted in our country, particularly on characterization of red mud and its use in the ceramic sector. [4, 10, 13, 14, 19, 20, 23-25, 27-32] While red mud composition is not fixed, it generally contains high amounts of Fe2O3 (30−60%), as well as Al2O3 (5−20%), SiO2 (1−20%), Na2O (1−10%) and TiO2 (trace − 10%). In addition, red mud may contain trace amounts of various impurities such as barium, copper, sodium, vanadium, chromium, nickel, lead, manganese, potassium and zinc. The high alkali content in red mud causes significant damage to the ecosystem. Particularly, its mixing into soil and groundwater causes serious problems. [33] For this reason, evaluation of red mud in application areas with large commercial volumes such as the insulation sector is quite important both for efficient use of natural resources and for minimizing environmental damage from industrial production.

Use of Ceramic Foam Materials in the Insulation Sector

Ceramic foam materials emerge as a valuable material group with significant application potential in thermal and acoustic insulation applications due to their excellent properties including lightness, high porosity, high surface area, low thermal conductivity, good thermal shock resistance and high chemical resistance. Compared to polymer-based foams used in the insulation sector, ceramic-based foam materials show higher mechanical, chemical and thermal stability. [34] In the event of a fire, polymeric foams used for insulation purposes are known to suffer significantly from toxic gases released during fire, while ceramic and glass-ceramic foams are non-flammable due to their inorganic nature. [35] Glass-ceramic foam production was first performed in the 1930s by introducing gases into molten raw materials, a method not much preferred today. Today, many different production methods can be used for production of porous ceramic and glass-ceramic materials. One of the simplest production methods for porous ceramics is sintering of starting powders with the addition of foaming agents. With the addition of foaming agents to the system, gas generation is provided during reactions occurring at high temperatures, thereby providing desired porosity in the structure. [36] During sintering, foaming can be achieved through oxidation or decomposition effects. For example, carbon-based foaming agents such as graphite, carbon black, coal, SiC, etc. undergo oxidation, and gas evolution during this process creates porosity in the structure. On the other hand, porosity in the structure can be achieved through gases released during decomposition of minerals such as carbonates (Na2CO3, CaCO3, MgCO3, etc.) or sulfates (CaSO4, plaster).

Production of Ceramic-Based Foams Using Red Mud

Ceramic and glass-ceramic foam materials can be produced using many different starting materials and different methods. A review of the literature shows a clear increase in scientific studies regarding the use of red mud, which appears as solid waste in industrial production processes, particularly in recent years for the production of ceramic and glass-ceramic foam materials. [34, 37-44] Guo and colleagues [34] produced glass-ceramic foam materials using the sintering method with red mud and fly ash as starting materials together with CaCO3 foaming additive. In this study, in compositions where the red mud/fly ash ratio did not exceed 2:3, foam materials with very low density (0.33–0.41 g/cm3) were able to be produced at low sintering temperatures (760–840 °C). Process variables such as sintering temperature and time applied during production were found to have significant effects on the compressive strength of the foams. Depending on process variables, compressive strength values of 0.33–2.74 MPa were obtained. Chen and colleagues [43] produced foam ceramics using the sintering method with fly ash and red mud as main raw materials. Small amounts of sodium borate were used in the composition to facilitate sintering, and sodium silicate was used to provide foaming. The study examined the effects of the ratio of red mud/fly ash and sodium borate and sintering temperature on the strength, bulk density, water absorption, microstructure and crystalline phase composition of the produced foams. [43] The obtained results showed that a homogeneous microstructure consisting of large pores was obtained in a composition of 40−50% red mud, 26.25−40% fly ash, 15−20% sodium borate and 5% sodium silicate sintered at 900°C for two hours. These foams are reported to have porosity of 64.14−74.15%, compressive strength of 4.04−10.63 MPa, flexural strength of 2.31−8.52 MPa, bulk density of 0.51−0.64 g/cm3 and water absorption of 2.31−6.02%. [43]

Figure 1. Scanning electron microscopy images of samples A1 (40% red mud, 40% fly ash), A2 (50% red mud, 30% fly ash) and A3 (60% red mud, 20% fly ash). [43]

Hou and colleagues [44] produced ceramic foam materials using the sintering method in their study using fly ash and red mud as main raw materials, sodium borate as sintering additive, and starch and MnO2 as foaming additives, sintering at 1000°C for 2 hours (Figures 2 and 3). As a result of the study, it was determined that compressive strength values increased with increasing foaming additive content. The obtained general properties are: bulk density of 0.59–0.96 g/cm3, water absorption of 3.16−9.17%, compressive strength of 4.22−8.38 MPa, flexural strength of 2.44−5.82 MPa, acid resistance of 95.59−99.60%, base resistance of 99.82−99.99%.[44] Due to these obtained properties, the produced foams are reported to have potential for use in the building sector. [44]

Figure 2. Scanning electron microscopy images of foam samples containing starch at (a) 1%, (b) 5% and (c) 15% ratios. [44]

Figure 3. Scanning electron microscopy images of foam samples containing MnO2 at (a) 1%, (b) 5% and (c) 15% ratios. [44]

Ceramic foams produced at different sintering temperatures using lead-zinc mine waste, silica sand and red mud as starting materials by Liu and colleagues [3] using the sintering method are presented in Figure 4. In this study, different pore formation mechanisms resulted in the formation of large quantities of closed pores. In the study conducted by Liu and colleagues [3], the ceramic foam sintered at 970°C had a density of 0.56 g/cm3, porosity of 76%, flexural strength of 5.3 MPa and thermal conductivity of 0.21 W/(m K). Due to their high porosity and low thermal conductivity, these foams are reported to be suitable for thermal insulation purposes.

Figure 4. Scanning electron microscopy images of samples sintered at (a) 900°C, (b) 950°C and (c) 1000°C. [3]

General Assessment

• Red mud, used together with different solid wastes such as fly ash, is an economical raw material that can be used in the production of ceramic and glass-ceramic foam materials for evaluation in the insulation sector. • These types of foam ceramics can be produced at low temperatures and in air using an economical method such as sintering that is easy to apply. • Red mud, fly ash, slag and marble waste, which are seen as problems in our country, have the potential to provide significant contributions to our national economy through their use in insulation material production in a large and rapidly developing sector such as building. • Additionally, through such applications it will be possible to minimize the damage that industrial solid waste currently causes to the ecosystem.   Dr. Ayşe Kalemtaş, Associate Professor Department of Metallurgy and Materials Engineering Faculty of Engineering and Natural Sciences Bursa Technical University  

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