## 1 Introduction  
Palm oil ash (POA) is an abundant agricultural solid waste in Malaysia. The ash is produced after the combustion of oil palm fiber and shell as boiler fuel to produce steam for electricity generation and palm oil extraction. Around 200 palm oil mills are in operation in Malaysia, where thousands of tons of POA are produced annually and disposed of without any commercial return [1]. The generation of palm oil ash (POA) and boiler ash (BA) in Malaysia were estimated to be over 4 million tons per year [2]. The generated waste could lead to health-related issues, environmental problems, and also financial loss [3; 4; 5]. Although there are some studies on the utilization of POA such as a partial replacement of cement in concrete [6], as an adsorbent for the removal of heavy metal from aqueous solution [7], flue gas desulphurization [8], and recently as a nano-structured material in polymer [9], most of the ash is still disposed of in landfill that requires a lot of land area. The utilization of POA in advance or conventional composites to reduce the cost, appearance, and properties of final products, has gained the attention of many researchers. Therefore, the utilization of POA becomes an essential topic to be further investigated. In order to obtain high-quality POA with fine particle size, homogenous size distribution, and unique morphology, various preparation techniques have been used to synthesize ultrafine POA, including ball milling and grinding technique [10; 11]. There has been an abundance of investigation dedicated to the production and characterization of nanoparticles due to their distinct characteristics, which are unavailable in conventional macroscopic materials. The mechanical ball milling technique is one of the well-known as a simple and low-cost method for the production of nano-structured materials [12]. This technique has proved to be an effective way to fracture and reduce the size of particles with the possibility of obtaining large quantities of nanomaterials. The process inside the ball mill involved mechanical energy transfer between the balls and the substrate was assured by continuous impact [13; 14]. Over the past decade, there have been several investigations to simulate the dynamics of the ball milling technique process in addition to analyse the parameters to produce nano-structured materials and to create the process more efficiently. The ball milling system consists of two rollers and one mill jar. The rollers rotate in one direction while the mill jar rotates in the opposite direction. During the rotation of the mill jar around its axis, the centrifugal force was created and helped the mixture of milling balls and powder. Thus, the impact energy of milling balls assists the friction of the powder into a smaller size. Many parameters have been tested to optimize the ball milling process, such as milling speed [15], milling time [16], balls size [17], and a ball to powder weight ratio [18]. Although many researchers have combined the parameters to determine the most influential parameter to produce nanoparticles, the results vary from one another. Therefore, there is no conclusive method on the best parameter applied to the ball milling process to transform macro size materials to nano-scale particles. The objective of this research is to design a process of transforming natural palm oil ash (POA) to POA nanoparticles by applying a set of parameter combinations in the ball milling technique. The traditional experimental design methods are too complex, time-consuming, and complicated to apply. Many experiments must be carried out when the number of process parameters is more. To solve this problem, the Taguchi method uses a special design of orthogonal arrays to study the entire parameter space with a minimum number of experiments [19]. This technique was used to produce a high content of POA nanoparticles, which is less than 100 nm (0.1 \(\upmu\)m) with optimization process from the Taguchi method analysis.