# Pyrophyllite: An Economic Mineral for Different Industrial Applications
## 1 Introduction  
Pyrophyllite is a hydrous aluminum silicate with the chemical formula Al\({}_{2}\)Si\({}_{4}\)O\({}_{10}\)(OH)\({}_{2}\) and is commonly associated with other minerals such as quartz, mica, kaolinite, epidote, and rutile [1; 2; 3]. The pure pyrophyllite is composed of 28.3% Al\({}_{2}\)O\({}_{3}\), 66.7% SiO\({}_{2}\), and 5% H\({}_{2}\)O on weight bases [4]. Pyrophyllite, when pure, is desirable for many applications due to its unique properties [5]. For example, pyrophyllite provides low thermal and electrical conductivity; high refractive behavior; a low expansion coefficient; high corrosion resistance; low bulk density; and low hot-load deformation [6]. Therefore, this mineral is widely used in the refractory, ceramic, fiberglass, pesticide, fertilizer, paper, paint, plastic, rubber, cement, building-material, and pharmaceutical industries [7; 8; 9]. Furthermore, since pyrophyllite has a lower coefficient of expansion and thermal conductivity compared to clay, pyrophyllite is suitable for refractory applications [10; 11]. Pyrophyllite, as a clay mineral, can be used as a substitute for kaolinite mineral group in many industrial applications, such as in the ceramic, and pottery, and filler industries, because kaolinite mineral group are rapidly depleting and expensive [12; 13]. Furthermore, pyrophyllite can replace talc in many applications, especially as a filler pharmaceutical and medical applications, since pyrophyllite is safe and free from associated toxic minerals such as asbestos [14]. Determining the market price of pyrophyllite depends on the mineral's Al\({}_{2}\)O\({}_{3}\) content and the presence of impurities (Table 1) [15]. Since pyrophyllite is formed via hydrothermal alterations, it contains impurities such as iron, titanium, and alkalis [16; 17]. The quality of pyrophyllite products is greatly affected by these impurities [18]. In addition, the presence of iron (Fe) and titanium (Ti) in pyrophyllite causes discoloration of the final product [19]. Accordingly, when pyrophyllite is used in the ceramic industry, this mineralstains the product's surface due to the presence of these impurities [12; 20]. Moreover, small amounts of iron lead to a decrease in the melting point of the refractory materials, affect the transparency of glass products, and reduce transmission in optical fibers [1; 3; 8]. Therefore, the most desirable percentage of iron in the pyrophyllite ore for use in industrial applications was determined in a previous study [8]. For example, in the refractory industry, the ore should contain less than 1% iron; in pottery and tile manufacturing, the ore should contain less than 0.5% iron; and in the paper industry, the ore should contain less than 1% iron [21]. It should be noted that alkalis are also deleterious because alkalis in the form of carbonates combine with silica (SiO\({}_{2}\)) under firing temperatures and form silicates that are soluble in water [3]. The presence of these silicates in products is not desirable, especially in the ceramic industry. Furthermore, alkali ions are electrically conductive [22; 23]. In 2017, a survey of the resource market showed significant growth in pyrophyllite production. High-grade pyrophyllite is produced in Korea, Japan, and China. Moreover, Japan and South Korea contribute more than 5% of the pyrophyllite produced globally [16; 27]. Table 2 shows the world production of pyrophyllite (By Principal Countries) from 2015 to 2019. The increasing demand for high-grade pyrophyllite ores with low impurities for industrial applications led to a scarcity in the reserves of such ore [5; 6]. Despite the presence of large quantities of low-grade pyrophyllite ores, the impurities in these ores limit their use. Thus, there is an imperative need for techniques to improve the purity of these low-grade ores [28]. Several techniques have been tested to remove impurities from industrial minerals, such as removing iron from dolomite [29], talc [30], and kaolin [31; 32].