Mineral-Based Pigments

1. Introduction

Pigments are organic and inorganic materials used to impart color and opacity to many products. Pigments consist of very small and insoluble particles that give color to the substance to which they are added, and are also used in obtaining other properties such as corrosion and heat resistance and microstructure reinforcement. This study will focus particularly on inorganic pigments, also known as "mineral pigments". Inorganic pigments can generally be said to have 3 functions: • Color provision, • Opacity (coverage), • Durability. The sectors where these types of pigments are most used are primarily paints, followed by paper, plastics, construction, ceramics and inks, textiles, glass, food and cosmetics. According to consumption volumes, white pigments (especially Titanium Dioxide, TiO2) and black pigments dominated by carbon black are the most consumed pigments. Colored pigments are relatively more costly. The main properties sought in mineral pigments are as follows: Appropriate color, opacity, oil absorption (absorption/retention) capacity, absence of water-soluble salts, moisture content, minimum Fe2O3 ratio, light resistance and reasonable price. On the other hand, the paints sector is the field where most of the world's TiO2 production—60% of global output—is consumed. White-colored pigments are used particularly in decorative paints sold on the market. Colored pigments are used in relatively much lower proportions and as additives. TiO2 consumption can be reduced by the addition of various filler minerals such as natural (GCC) and precipitated/synthetic (PCC) calcium carbonate, kaolin and talc. White pigments (TiO2 and other filler minerals) are preferred in the plastics and paper industries just as they are in paints. Pigments of different colors are also used in plastics. In the construction sector, oxide pigments are generally preferred because they are relatively lower in cost, alongside manganese and certain other compounds.  

2. Natural (Mineral) Pigments

Pigments can generally be divided into 3 groups based on their chemical structure: natural inorganic, natural organic and synthetic. This research will address only those of natural inorganic (mineral) origin.

2.1. Iron Oxide

These minerals can be grouped under four headings according to their colors: yellow, red, brown and black. Iron oxides can also be produced in a wide variety of colors through blending. Commercially, they can be classified as follows: paint earth-ochre (red, yellow), amber pigment (dark brown), siena earth (yellow, orange) and red iron oxide. In natural pigments, the coloring element is generally the iron element within minerals. Minerals such as hematite, magnetite, goethite and limonite form yellow-red and brown colors such as ochre and amber through decomposition and weathering. While hematite gives a red color, magnetite can vary from brown to black. In general, paint earth is rich in iron, poor in manganese and can give colors ranging from yellow to various shades of red. These occur through decomposition of iron deposits and clayification of iron-rich rocks such as diobase. Siena earth similarly has a color range from yellow to orange, while amber is dark brown due to its iron and manganese content. Mineral pigments can have their colors deepened and stabilized by calcination. [caption id="attachment_144434" align="aligncenter"] Table 1. Iron oxide pigment sources and colors[/caption]

2.1.1. Yellow

Yellow paint earth is characterized by relatively low iron oxide levels (20-50%). Red paint earth contains higher iron oxide content. Siena is distinguished from paint earth by its high iron oxide content (60%). Like paint earth, sienas also occur in various shades of yellow.

2.1.2. Brown

The main source is amber. Amber has a yellow-olive green color due to the presence of yellow-colored hydrated iron oxide and black-colored manganese compounds. Calcination of amber at 260-425°C removes water content and changes its color. Depending on iron oxide content it transforms from yellow to red, and the black manganese compounds also produce a brown walnut-colored product.

2.1.3. Red

The main source of red iron oxide pigment is hematite (Fe2O3).

2.1.4. Black

Natural black iron oxide is obtained from magnetite (Fe3O4). However, it has larger particle size compared to synthetic varieties and weaker color-providing (dyeing) properties.  

2.2. Chromium Oxide

Chromium oxide (Cr2O3) pigments are also known as "chrome green" pigments. Chrome green is a mixture of chrome yellow and iron blue pigments, while phthalochrome green is a mixture of chrome yellow and blue phthalocyanine pigments. Chromium oxide green (light-dark) pigments are insoluble in acidic, aqueous and alkaline environments and are stable in the presence of sulfur dioxide and in concrete. They are resistant to light, atmospheric conditions and temperature and are used in infrared camouflage coatings. On the other hand, in glass manufacturing, the addition of green chromium oxide turns glass a green color (for example, soda bottles). The higher the concentration, the more intense the color. A commercial example of this is wine bottles. It also provides some light protection to preserve the taste of wine. Green chromium oxide is also used in the production of refractory (fire-resistant) bricks due to its melting point of 2,300°C. It is also used in industrial paints where excessive wear occurs. For example, military vehicles are painted with green chromium oxide paint. When exposed to high heat and light, it maintains its color and is an ideal choice for outdoor applications. It not only adds color to inks, eyeglasses and paints, but also gives green to plastics and ceramics. It mixes well with clay and can be used as a component in varnishes.  

2.3. Manganese Oxide

When considered in terms of manganese content, total world manganese production is approximately 6 million tons. However, only a very small portion of this is used in the pigment industry. Besides mining methods, manganese is also produced through electrolysis and chemical processes. Most of the manganese used in pigment-grade material is ground ore. Manganese violet-colored pigment (manganese ammonium phosphate) is produced in small quantities. Manganese blue (barium sulfate permanganate) can also be produced. However, all manganese pigments used are in the oxide MnO2 form (produced by heat decomposition). This generally means a very dark color close to black. However, it can also contain red, brown, purple or black tones. Manganese pigments are generally used to produce black, brown and gray colors and contain 65-80% manganese dioxide (MnO2) as pigment.

2.4. Mixed Metal Oxide

The term mixed metal oxide pigment refers to pigments that have crystallized in a stable oxide lattice. Color is produced by the combination of cations in the lattice. The three mixed metal oxide pigments used as pigments are those with rutile (TiO2), spinel (MgAl2O4) and hematite (Fe2O3) structures. Stable oxide lattices have very good chemical/thermal stability and good optical pigment properties with high refractive index.  

2.5. Mica

Although many minerals are defined as mica, the most commercially important is muscovite mica (white mica). The others are not commonly used in filler or pigment applications. However, phlogopite is used in some special applications. Only a small portion of mica production is used in pigment applications. Before use, mica is ground into various sizes and used as a fine-grained and flaky pigment raw material. Micronized mica (d50 < 53 microns) is normally used in pearlescent pigment production. Mica selection is an important factor in determining the quality and appearance of the pigment. Aspect ratio of 300-600 nanometers thickness and mica sheets in different size ranges (5-25 microns, 10-50 microns and 30-110 microns) depend on particle size distribution. It is also possible to produce different combinations with other pigments. Another recent development is absorbing pigments such as graphite, laminar phthalocyanine and flaky iron oxide. Iron oxide layers exhibit metallic color effects and various tones. These pigments are generally preferred to provide metallic paint effects. They are used in various fields from plastics to printing inks, from automotive topcoat paints to cosmetics.

2.6. Zircon

Zircon pigments are used in the ceramics industry due to their resistance to high temperatures during firing. More than half of all colored pigments used in glazes and wall tiles are accounted for by these pigments. Different colors can be obtained by mixing zircon structure with small amounts of other elements. For example, vanadium provides blue color, while praseodymium (Pr) provides light yellow and iron provides coral pink color. Until recently, zirconium oxide was obtained either from naturally occurring baddeleyite (ZrO2) or much more commonly from zirconium silicate (zircon, ZrSiO4). While reformulation of special quality is possible without using baddeleyite, in most cases a complete reevaluation of the raw materials that produce color for ceramic pigments is necessary.

2.7. Rare Earth

The most common rare earth pigment is cerium sulfide (CeS), a red pigment. It can be produced in colors ranging from orange to burgundy. Praseodymium (Pr) is also used for pigment applications, but is essentially used as an additive in zircon pigments to produce yellow ceramic pigment. Neodymium (Nd) can be used in obtaining purple color, while yttrium (Y) can be used as an orange pigment. Cerium oxide (CeO2) is used as a matting agent in white ceramic glazes, but for this type of application the base material is zircon due to it being less expensive. [caption id="attachment_144435" align="aligncenter"] Table 2. Various pigment sources and
colors obtained[/caption]

3. Conclusion

Pigments are materials produced through organic, inorganic or synthetic methods that provide color, opacity and durability in many different fields. Particularly mineral-based pigments are commonly used in paints after being subjected to various production processes.   References [1] Akar, A., 1987. Endüstriyel Hammaddeler ve Zenginleştirme Yöntemleri, DEÜ yayını, MM/MAD-87 EY 142, İzmir. [2] Dickson, T., 2006. The Colour of Minerals, Industrial Minerals, September 2006, p.28-35. [3] Kayaalp, M.N., 2010. Siyah Pigmentler, BoyaTürk, Şubat-Mart 2010, s.80-84. [4] Milli Eğitim Bakanlığı, MEGEP (Mesleki Eğitim ve Öğretim Sisteminin Güçlendirilmesi Projesi), Kimya Teknolojisi, Pigmentler, 2008, Ankara. [5]Toraman, Ö.Y., Çiftlikli, M. 2012. Doğal (Mineral) ve Sentetik Pigmentler: Sektöre Genel Bakış, BoyaTürk, Nisan-Mayıs 2012, s.48-56. [6] http://www.madencilik.net/hm_boya/oksitler.doc [7] http://www.reaksiyon.com.tr/index_organik.html [8] https://www.kimyaborsasi.com.tr/tr/k/krom-oksit-75.html     Prof. Dr. Öner Yusuf Toraman Niğde Ömer Halisdemir Üniversitesi Mining Engineering Department