Industrial minerals and rocks
According to the Pan-European Reserve and Resources Reporting Committee Standard (2018), natural mineral raw materials are divided into metallic, non-metallic (industrial minerals and rocks) and energy. These terminologies describe, in general, the physical properties and the use of these mineral raw materials. Ore minerals (native metals and metal compounds) belong to the first category, while industrial minerals and rocks, as well as energy raw materials (liquid, solid, gaseous) belong respectively to the second and third categories.
Following the rapid technological growth, new uses for industrial minerals and rocks have found industrial application as raw materials and as additives. Thus, for many industrial minerals and rocks such as talc, perlite, bentonite, clay, marbles, shale, limestone, and sand, new uses have been developed. While others, such as baryte, fluorite, and apatite, were considered critical mineral raw materials, as they serve as sources of barium, fluorine, and phosphorus, elements whose undisturbed supply to the modern developed industry is considered to be of strategic importance. But the role of these mineral raw materials in the environmental industry is also important. In recent years, the importance of the use of industrial minerals and rocks in environmental protection applications has become more and more prominent, such as the use of perlite in treating oil spills and marine pollution, and clay minerals in waterproofing of waste disposal sites.
Zeolite belongs to the category of the industrial minerals and rocks, and holds a leading role between the industrial minerals and rocks used in the environmental industry. Below, on the basis of the representative literature, follows a brief description of the physical and chemical properties of the mineral, which give to it its unique characteristics.
Figure 1. (a). Different arrangements of the zeolites crystalline structures, (b). The structural geometry of the micro- and nano-pores observed in the zeolites.
Zeolite characteristics
In fact, the term zeolite does not refer to solely one mineral and the singular voice is used for the sake of simplicity. The term, therefore, refers to a group of minerals, the zeolites. The minerals constituting this group have a similar chemical composition and have Na, Ca, K, Ba, Sr, and hydroxyl (OH) as their main chemical elements. The Swedish mineralogist Cronstedt in 1976 first discovered the zeolites group and named it by composing the ancient Greek words «ζέω» “bloom” and «λίθος» “stone”. His choice of words was based on their characteristic ability to release bubbles when being heated. This is explained by the crystalline structure of zeolite which exhibits empty spaces (Figures 1a, b). These spaces are filled with water molecules and free cations loosely connected to each other. When zeolite are heated up to 350 º C this water boils and is discharged without destroying the crystalline structure of the mineral (Fig. 1a). In continuity, the water re-adsorption in the empty spaces of the crystalline structure is not excluded. Another characteristic feature of the zeolites group minerals is the ability of reciprocal ion exchange in their molecular matrix.
The general chemical formula of the zeolites is: Μ2/nO.Al2O3.xSiO2.yH2O, where: M = alkali or alkaline earth metals, n = cation valence, x = 2 to 10, and y = 3 to 7. The most widespread members of the natural zeolites group are: analcime (Na16(Al16Si32O96).16H2O), heulandite ((Ca,Na)2-3Al3(Al,Si)2Si13O36·12H2O), chabazite ((Na2Ca)6(Al12Si24O72).40H2O), clinoptilolite ((Na4K4)(Al8Si40O96).24H2O), laumontite (Ca4(Al8Si16O48).16H2O), and natrolite (Na16(Al16Si24O80).16H2O). Overall, 67 minerals of natural zeolites are known, while more than 100 different synthetic zeolites have been formed in the laboratory conditions.
Geologically the formation of zeolite group minerals is associated with the hydrothermal erosion of volcanic rocks (e.g. volcanic tufts) through the circulation of hot water in the rocks through faults and cracks. Therefore zeolites are secondary minerals and can also be found in sedimentary rocks (e.g. grauwacke, mineral coals and ferrous sedimentary formations). Zeolite formation conditions are mainly characterized by: low partial CO2 pressure, high pH, and increased presence of Na, Ca, K, SiO2 and H+ in circulating aqueous solutions. After the middle of the 20th century, intensive geological surveys, especially in volcanic areas, have discovered new stocks of zeolites. This is rather important as synthetic zeolites are comparatively more expensive (preferred only for specialized industrial uses) from the exploration, exploitation and production of natural zeolites.
The volcanic rocks mainly bearing large amounts of zeolites are tuffs. Although, except zeolite grpous minerals, zeolitic tuffs may contain also other minerals (usually 4 to 10 minerals) such as micas, smectite, illite, celadonite, quartz, christobalite, tridymite, opal, alkali-feldspar and plagioclase. Among the various types of zeolithic tuffs the HUE-type (clinoptilolite-heulandite) zeolite bearing tuffs can found the most industrial applications. However, according to Filippidis & Tsirambidis (2015) they must be characterized by the following; (1) zero presence of fibrous zeolites (2) more than 75% of clinoptilolite-heulandite, (3) the main and trace chemical elements contents should not exceed the maximum limits set by legislation, (4) do not contain swelling clay minerals, and 5) less than 14% of non-microporous minerals (e.g. quartz) and more than 86% of microporous minerals (HEU-type zeolite + mica + clay minerals). In Greece there are found 68 locations of natural zeolites of different qualities, while the reserves are estimated at 600 million tons of a total value of approximately € 18 billion.
Applications of zeolites
The basic physical and chemical properties of the zeolites, which rank them among the most environmentally friendly minerals, are the result of their fine chemical composition and crystal structure, resulting in their unlimited applications. These physicochemical properties are described as sorption and fixation and are active within the ctrystalline structure of the mineral in nano-, micro-, meso-, and macro-levels. Another important fact of their nature is the absence from their chemical composition of dangerous elements, such as heavy metals.
In industrial products, where the zeolites are used as components, their use provides properties to the final products such as: high strength, abrasion resistance, reduced weight in relation to volume, high water and moisture absorption capacity, containment of odors, better control of the availability of metals and nutrients in soils, and better oxygenation of confined spaces. Thus, zeolites are finding extensive applications in the building, livestock and agricultural sectors, as well as in the industries of detergents, paper, energy, drying, insecticide and pesticide, polishing materials, catalysts and special filters.
The Petrota natural zeolite
In the area of Petrota, clinoptilolite is the main zeolite group mineral found in the mineralogical composition of the volcanic tuffs, along with minor and only locally occurring mordenite, something of particular attention. Several scientific reports highlight that a final product of a certain percentage of clinoptilolite per weight could be used as: as food additive for domestic animals, in agricultural crops, and in wastewater treatment plant. Although the presence of mordenite in zeolitic products intended for human or animal usage is strictly prohibited, some uses are mentioned worldwide as oil-absorbing material, as soil improver, and as aggregates.
Zeolite is an environmentally friendly mineral that does not contains dangerous components and heavy metals in its composition. With the proper attention to the local landscape architecture, and by taking into consideration the Environmental Law regarding the quarrying processes, the Petrota zeolite could be sustainably quarried supporting the demographic and economic growth of the broader region.
Representative References (in Greek & in English)
Τσιραμπίδης Α., 1996, Τα Ελληνικά μάρμαρα και άλλα διακοσμητικά πετρώματα. University studio press, Θεσσαλονίκη, σ. 310.
Βαβελίδης Μ., Χοτζίδης Α., και Μέλφος Β., 2007. Λατομεία και λατόμοι στη Θράκη. Στοιχεία για την αρχαία και σύγχρονη εποχή, Πρακτικά Ημερίδας: Δυνατότητες ανάπτυξης στο Βόρειο Έβρο: Πολιτισμός, ορυκτοί πόροι και περιβάλλον, Πετρωτά Έβρου, 4/8/2007.
Φιλιππίδης Α., 2007, Ζεόλιθοι Δήμου Τριγώνου του Νομού Έβρου στη βιομηχανική, αγροτική, κτηνοτροφική και περιβαλλοντική τεχνολογία, Πρακτικά Ημερίδας: Δυνατότητες ανάπτυξης στο Βόρειο Έβρο: Πολιτισμός, ορυκτοί πόροι και περιβάλλον, Πετρωτά Έβρου, 4/8/2007.
Filippidis A., 2010, Environmental, industrial and agricultural applications of Hellenic Natural Zeolite, Hellenic Journal of Geosciences, v. 45, p. 91-100.
Τσιραμπίδης Α., Φιλιππίδης Α., 2013, Ορυκτοί Πόροι Ελλάδος: Αποθέματα και Αξία, Τμήμα Γεωλογίας, Σχολή Θετικών Επιστημών, Αριστοτέλειο Πανεπιστήμιο Θεσσαλονίκης, σ. 46.
Φιλιππίδης Α., & Τσιραμπίδης Α., 2015, Μάρμαρα και Ζεόλιθοι: Ποιοτικά χαρακτηριστικά – Αποθέματα και αξία. Βιομηχανικές, περιβαλλοντικές και αγροτικές εφαρμογές. Επιχειρηματική Ανακάλυψη της Αλυσίδας Αξίας των Μη Μεταλλικών Ορυκτών στην Ανατολική Μακεδονία και Θράκη, Επιχειρησιακό Πρόγραμμα «Μακεδονία-Θράκη» 2007-2013, ΕΣΠΑ, 5/5/2015, Δράμα, σ.12.
Pan-European Code for Reporting of Exploration Results, Mineral Resources and Reserves, http://46.32.234.2/perc/documents/PERC_REPORTING_ CODE_jan2009.pdfwww.perc.co, (τελευταία επίσκεψη 15/7/2018).