## 1 Introduction  
Fly ash is one of the byproduct produced from the coal-fired power plant through the combustion process for power generation. Mae Moh coal-fired power plant (Lampang province, Thailand), the biggest one in the country, producing a large amount of byproducts each year, including bottom ash, fly ash, and gypsum [1; 2; 3; 4]. Mae Moh lignite fly ash having good pozzolanic and binding properties enabling it an excellent choice mainly for construction [3; 5; 6]. In Thailand, the most effective use of fly ash is in the area of concrete [7; 8; 9; 6; 10]. Fly ash is known to be used to substitute Portland cement Type 1 [11], which can reduce production costs while increase the product properties. Over decades, lignite fly ash has been used in many important construction industries, such as concrete pipe and pile manufacturing and in paving blocks [7; 10]. The large-scaled construction projects include Khun Dan Prakarn Chon Dam in Nakhon Nayok, Rama 8 Bridge's abutment, MRT underground tunnels in Bangkok, footings of the Suvarambhumi International Airport, and also construction abroad in the Xayakuri Dam and Nam Theun 1 Dam project in Laos [12]. Considering the volume of fly ash and its value as concrete, fly ash is a cement-substituting material used to improve concrete properties, such as durability, improved workability, and long-term strength. The Thai lignite fly ash has high calcium; the high calcium content has pushed fly ash to the utilization limit in various mass applications [3; 5; 6]. According to the concept of the circular economy focusing on resource reuse and recycling, focusing maximizing the efficient use of existing resources, minimizing the waste--thus reducing the environmental impact--are principal concerns of the power plant. A value-added material or a subproduct mixing with fly ash is cenospheres. Cenospheres are hollow, spherical shaped particles, mostly open-pore in nature, and a lightweight material. Cenospheres are of a similar chemical composition to fly ash as their mineral composition is dependent upon the geological and chemical features of coal source and the combustion conditions forming them [13; 14; 15]. Cenospheres have superior properties are lightweight, chemically inert and have good flowability, improved workability, good insulation, good thermal resistance, and excellent mechanical strength [16; 17; 18]. With their distinctive properties, they have become a highly in-demand material in many specialized applications, such as light weight additive or filler in construction, cement and concrete [19; 20; 21; 22], polymer composites [23; 24; 25], fabricated composite in railway systems [26], sound-absorbing material [27], thermal insulation products [17; 28; 29; 30], composite for metal matrices [31], and ceramic foam [32; 33]. Cenospheres offers significant advantages in a wide range of industrial processes, including manufacturing, product improvement, and cost reduction purpose. Utilizing nanospheres in the products, performance relating to properties of material depends considerably on the cenospheres characteristics, which include physical properties, morphological structure, and chemical composition. A good understanding of cenosphere characteristic properties is considerably essential to not only the optimization of cenospheres recovery parameters, but also the development of effective strategies to recover cenospheres with a controllable quality. The methods of cenosphere separation are wet separation and dry separation. For the wet separation in liquid medium, cenospheres can be generally separated from fly ash with the mechanism based on the density difference between particles and liquid. Wet separation is the method mostly used to separate lightweight or low-density cenospheres. The optimization of cenospheres recovery process to obtain high recovery yield (yet limited by their content by nature) and good quality of cenospheres, is important.