Production of glass-ceramic and ceramic materials from industrial wastes

Abstract

In the near future the development of new recycling technologies is getting more important and the recycling of by-products and industrial waste materials will dramatically increase. Fly ash, a waste product of coal combustion in thermal power plants, is produced in large quantities and thus is a major source for environmental pollution. In Turkey, considerable amounts of coal fly ash is generated daily in the thermal power plants, due to the high coal consumption and the high mineral matter content of Turkish lignites. The management of coal fly ash is a major problem in Turkey since only a small amount of it has been utilized. Fly ash contains various valuable oxides such as $(SiO_2)$, $Al_2O_3$, CaO, $Fe_2O_3$, and other oxides. These oxides have been mainly considered as a low cost material resource for the glass, glass-ceramic and ceramic industry. Red mud is another waste material generated by alumina production from bauxite. In this study, it is aimed to use coal fly ash, belonging to $SiO_2-Al_2O_3-Fe_2O_3$ ternary system and red mud, in the development of low-cost glass, glass-ceramic and ceramic materials. For this purpose, all glass and glass-ceramic materials were firstly produced from coal fly ash with the addition of 5% red mud. The crystallization activation energy of glass sample was determined as 370 kJ/mol by using Kissinger equation. To produce the glass-ceramic samples, sintering method was applied to the powder glass samples. glass samples were milled until they passes through a sieve of 180 $mu m$. Five wt % polyvinil alcohol (PVA) water solution was added to the grounded powder to determine the effect of the binder on the properties of sintered glass-ceramics. Glass powders were than cold pressed using 40 tons in a disc shape (10 x 5 mm). Cylindrical samples were dried in an electric oven at 383 K for 2 h. Finally, both bulk and pressed glass samples were crystallized by suitable nucleation and crystal growth heat treatments on the basis of DTA results. All the glass samples were heat treated at the crystallization temperature for 15, 30 and 60 min to investigate the effect of holding time at the crystallization temperature on the properties of glass-ceramic samples. X-ray diffraction (XRD) analysis of the sintered glass-ceramic materials revealed that the main crystalline phase was diopside (Ca(Mg,Al)$(Si,Al)_2O_6)$. Scanning electron microscopy (SEM) investigations showed that tiny crystallites homogeneously dispersed in the microstructure of the produced glass-ceramic samples. It was observed that addition of polyvinyl alcohol (PVA) as a binder and the increasing of holding time at the crystallization temperature improved the properties of the sintered glass-ceramic materials.Ceramic materials were also produced from coal fly ash without any additives and binders using classical sintering technique. In sample preparation, a small amount of water was used to humudify the fly ash before compaction. The circular pellets of 10 mm diameter were uniaxially pressed at 40 MPa to achieve a reasonable strength. The sintering temperature varied between 1298 K and 1373 K to determine the effect of firing temperature on the properties of sintered fly ash samples. The heating rate was 10 K/min and the sintering time was 120 min for all samples. XRD analysis revealed that quartz $(SiO_2)$, mullite $(Al_6Si_2O_{13})$, anorthite $(CaAl_2Si_2O_8)$, hematite $(Fe_2O_3)$ and enstatite ((Mg,Fe)$SiO_3$) phases detected in the fly ash sample disappeared; diopside (Ca(Mg,Al)$(Si,Al)_2O_6)$ phase occurred in the ceramic sample at the sintering temperature of 1373 K. SEM investigations were conducted on the produced ceramic materials to investigate the microstructural evolution of the samples. Inspection of crystalline structures of ceramic samples indicated that the spherical crystallites dispersed randomly in the microstructure of the samples. The number of the crystallites and the average crystalline size increased with the increase in sintering temperature. It was observed that the crystal size and morphology appeared to play an important role in affecting physical and mechanical properties of ceramic materials. The density and the hardness values of the ceramic samples increased significantly with increasing sintering temperature while the water absorption and porosity values decreased. Overall results showed that it is possible to produce glass, glass-ceramic and ceramic materials from coal fly ash. It was also concluded that produced glass-ceramic materials had superior properties than the obtained ceramic materials.

Özet

Bu çalışmada, termik santral uçucu külü ile alüminyum üretimi sırasında açığa çıkan kırmızı çamurdan cam ve cam-seramik üretimi gerçekleştirilmiştir. Öncelikle, cam üretimini gerçekleştirebilmek amacıyla, Çayırhan uçucu külüne %5 oranında atık kırmızı çamur ilave edilmiştir. Kissinger yöntemi kullanılarak, üretilen cam numunelerinin kristalizasyon aktivasyon enerjisi 370 kJ/mol olarak tespit edilmiştir. Cam numunelerine, cam-seramik malzeme üretebilmek amacıyla sinterleme yöntemi uygulanmıştır. Cam numunelerine; diferansiyel termal analiz sonucunda elde edilen bilgiler ışığında, çekirdeklenme ve kristalizasyon ısıl işlemleri uygulanarak cam-seramik malzemeler üretilmiştir. Kristalizasyon sıcaklığında bekleme süresinin üretilen cam-seramik numunelerinin mikroyapısal, kimyasal ve fiziksel özelliklerine olan etkisi incelenmiştir. Kristalizasyon sıcaklığında süresinin artması ile birlikte cam-seramik numunelerde oluşan kristal fazın oranı artmıştır. Ayrıca cam-seramik numunelerinin iyi yönde geliştiği gözlenmiştir. Sinterleme yöntemiyle üretilen cam-seramik numunelerde oluşan kristalin fazın diopsid (Ca(Mg,Al)$(Si,Al)_2O_6)$ olduğu belirlenmiştir. Ayrıca, uçucu külden herhangi bir katkı maddesi ilavesi olmadan, dört farklı sinterleme sıcaklığında ısıl işlem uygulanarak seramik malzemeler üretilmiştir. Üretilen seramik numunelerde oluşan fazların kuvars $(SiO_2)$, mullit $(Al_6Si_2O_{13})$, anortit $(CaAl_2Si_2O_8)$ ve diopsid (Ca(Mg,Al)$(Si,Al)_2O_6)$ olduğu saptanmıştır. 1348 K’in üzerindeki sinterleme sıcaklığında üretilen seramik malzemenin, daha düşük sinterleme sıcaklıklarında üretilen numunelere oranla daha üstün özelliklere sahip olduğu tespit edilmiştir. Üretilen cam-seramik malzemelerin mikroyapısal ve fiziksel özelliklerinin, seramik malzemelere kıyasla çok daha iyi olduğu gözlenmiştir.