Crystallization evolution, microstructure and properties of sewage sludge-based glass–ceramics prepared by microwave heating

Abstract

A Microwave Melting Reactor (MMR) was designed in this study which improved the microwave adsorption of sewage sludge to prepare glass–ceramics. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used for the study of crystallization behavior and microstructure of the developed glass–ceramics. DSC and XRD analysis revealed that crystallization of the nucleated specimen in the region of 900–1000 °C resulted in the formation of two crystalline phases: anorthite and wollastonite. When the crystallization temperature increased from 900 to 1000 °C, the tetragonal wollastonite grains were subjected to tensile microstresses, causing the cracking of crystal. Al ions substituted partially Si ions and occupied tetrahedral sites, giving rise to the formation of anorthite. The relationship between microwave irradiation and crystal growth was studied and the result indicated that the microwave selective heating suppressed the crystal growth, giving apparent improvements in the properties of the glass–ceramics. The glass–ceramics products exhibited bending strength of 86.5–93.4 MPa, Vickers microhardness of 6.12–6.54 GPa and thermal expansion coefficient of 5.29–5.75 × 10⁻⁶/°C. The best chemical durability in acid and alkali solutions was 1.32–1.61 and 0.41–0.58 mg/cm², respectively, showing excellent durability in alkali solution.

Introduction

One of the main environmental problems is the safe disposal of the huge amount of sewage sludge that is produced every day in wastewater treatment plants [1]. Among the methods of the treatment of sewage sludge, glass–ceramics preparation seems to be a promising one for converting sewage sludge into novel materials that possess attractive mechanical and chemical properties [2]. Sewage sludge containing large amounts of CaO, SiO₂, and Al₂O₃ can be a good raw material for glass–ceramics production. By controlling the initial composition and by suitable heat treatment, a variety of crystalline phases will be obtained [3]. They exhibit bending strength, Vickers microhardness, fracture toughness, chemical durability and thermal shock resistance superior to those of glass, and in some cases traditional ceramics [4], [5]. It should be noted that the chemical energy of the organic components in sewage sludge could be recovered during the prepared procedure of glass–ceramics as an auxiliary energy source. The chemical energy in sewage sludge can be recovered during the prepared procedure of glass–ceramics as an auxiliary energy source, reducing the emission of CO₂ which is favorable to Kyoto Protocol [6]. Other advantages of this technology are the possibility of immobilizing heavy metal ions (held in the framework of glass or encapsulated into the crystallization phase) [7], the large reduction of volume (vary between 40 and 90%), and the flexibility of treatment procedure (which may accept different types of sewage sludge, either municipal or industrial) [8]. Preparing glass–ceramics by the conventional technology is an energy-intensive process, with the process temperature as high as about 1300 °C and the process time required as several hours [9]. Economic analysis of a glass–ceramics preparation system which can process 0.5–1.0 ton of sewage sludge per hour showed that the operating costs of this unit ranged from US$100–420 per ton, including labor, fuel and maintenance [10]. Another critical point in glass–ceramics preparation is the difficulty in controlling the size and the type distributions of the crystals due to the thermal inertia of the conventional heating [11].

To overcome these drawbacks, microwave heating has been developed as an alternative technology for the preparation of dense structural glass–ceramics, which is characterized by shorter reaction time, reduced energy consumption, and suppressed crystal size. It was found that the treatment temperature was decreased from 1300 °C to 1000 °C when a glass–ceramics was sintered from barium aluminosilicate glass in microwave field [12]. It was also reported that an abrasion resistant glass–ceramics was developed from the MgO–Al₂O₃–TiO₂ system in 20 min by microwave heating [13]. Moreover, more uniform and strong bonding was observed in the glass–ceramics prepared by microwave, indicating that microwave energy suppressed the grain growth in crystal phase due to a fast heating rate and apparent low-temperature crystallization [8].

Sewage sludge is a poor receptor of microwave energy to achieve the temperature necessary for preparing glass–ceramics. It has been proved that microwave-induced preparation is possible, if an effective receptor is added into the raw sludge. The temperature of sewage sludge can achieve 1200 °C in microwave field when it was homogeneously blended with microwave receptor, such as graphite and char [9]. However, there are fundamental disadvantages of this method when it is applied in glass–ceramics preparation. The chemical composition of the samples shows uncontrollable changes in virtue of adding microwave receptor, leading to the poor properties of the products. In addition, the microwave receptor could not be recovered due to the encapsulation of silicate matrix in the glass–ceramics, increasing the operating cost of the procedure. Attempts termed as “hybrid microwave sintering” were also made to set around the sample directly to initially heat the material at room temperature [14]. However, the temperature of sewage sludge could not reach high enough owing to the significant reflection loss on the interface between microwave receptor layer and the air surrounding it.

To solve these problems, a new Microwave Melting Reactor (MMR) was designed in this study for preparing glass–ceramics from sewage sludge. In MMR, microwave absorption of sewage sludge can be improved by the double-layer structure and the required temperature can be achieved in a very short of time, usually in a few minutes. A wave-transparent layer was introduced into the MMR system to decrease the reflection coefficient of the interface between the air and the MMR. Another important property of the powder was the low thermal conductivity which could give the sample a good heat insulation quality. The double-layer structure in MMR provides the even distribution of temperature and electromagnetic field in the samples, favoring the production of glass–ceramics with desired qualities. Further researches presented in this paper were focused on: (1) investigating the influence of heat-treatment schedule on the crystallization behavior and microstructure of the microwave-prepared glass–ceramics, (2) defining the evolution of crystallization in microwave field which were hardly found by applying the conventional procedures, and (3) gaining an insight into the chemical and physical properties of the glass–ceramics prepared by microwave in comparison to that obtained from conventional process.