Summary:
Kyanite polymorph minerals are widely used in the refractory industry due to their excellent properties under high-temperature conditions. Furthermore, their ability to form mullite-rich aggregates, a high-grade refractory mineral, after thermal decomposition, makes them ideal for applications in high-temperature furnace linings and vessels in the iron, steel, cement, ceramics and glass industries. However, the compatibility of kyanite minerals with refractory uses is defined by specific industrial standards. The current MSc thesis aims to characterize the kyanite occurrences of Naxos (Greece) according to their potential of exploitation for refractory purposes. To accomplish that, the following tasks were performed: (a) locate and map the kyanite-bearing rocks and recover representative samples, (b) identify the mineralogy of kyanite host-rocks, the textures and relation between kyanite and barren minerals, (c) produce kyanite concentrates through gravity separation and estimate the kyanite content of mica-schists, (d) examine the concentrates’ quality concerning industrial standards by mineral and chemical analysis and (e) test the refractory behavior of kyanite concentrates and their ability to form mullite aggregates through different stages of calcination.
Three areas were located in central Naxos, near the villages of Moni, Sifones and Skado, where outcrops of kyanite-bearing schists protrude through thick vegetation, farming fields and cliffs, along an approximately parallel strike to the west contact of schist and marble rocks. Westward of Skado, near the summit of mount Mavrovouni (at an elevation of 996 m), kyanite schists are found near the contact of schist - marble and amphibolite rocks (Naxos’ migmatite). The majority of schist blocks featured NE dipping at a 60° to 65° angle, striking N060°E. In northern Naxos, northwest of Apollonas, aggregates of bluish/white kyanite crystals (10 cm length) were observed, hosted by quartz/aplitic veins (up to 0.5 m width). All kyanite mother-rocks mapped were in the vicinity of corundum lenses hosted by marble rock.
21 samples were collected, 19 from kyanite-bearing schists, and 2 from vein-kyanite occurrences near Apollonas, which were analyzed by X-Ray Diffraction (XRD). The mineral paragenesis of the schist samples is identical and consists of quartz, biotite, muscovite, albite, kyanite ± sillimanite. The mineralogy of vein samples is similar, but with higher peaks for kyanite’s, muscovite’s XRD patterns and lower for biotite’s, and the replacement of albite by anorthite. Eight (8) thin sections were examined through Optical Microscopy and Scanning Electron Microscopy - Energy Dispersive Spectroscopy (SEM-EDS), two for each study area (a schist and a vein sample from Moni and Skado, two schist samples from Sifones and two vein samples from Apollonas). 331 EDS semi-quantitative microprobe spot analysis and over 400 backscattered electron and optical microscopy images were produced to determine the close genetic relationship of kyanite with mica (biotite, muscovite) as kyanite crystals often “float” in mica zones and kyanite cracks or pores are filled by mica minerals. Also, cases of mica and quartz pseudomorphs have been observed, as a concequence of kyanite crystals’ alteration. The majority of kyanite crystals analyzed showed ideal Al2O3/SiO2 ratio without elemental impurities, absence of mineral inclusions and massive to schistose texture. In cases of massive kyanite crystals, where cleavage is not visible, the only way to distinguish kyanite from quartz is via EDS microprobe spot analysis. Exceptions have been observed where mineral inclusions exist in kyanite crystals, most of which found in Moni’s samples. These inclusions consist of: biotite, muscovite, quartz, apatite, zircon, monazite, xenotime, ilmenite and goethite. Rare Earth Element (REE) minerals (monazite, xenotime), possibly of detrital origin, are hosted mainly by mica group minerals. Vein kyanite samples of Apollonas contained monazite and xenotime up to 100 μm in length. Liberation size (500-280 μm) of kyanite crystals was also concluded by microscopy observations and existing literature on the topic. Metallurgical processing of schist samples aiming to the recovery of kyanite concentrates included crushing, dry screening and gravitational separation, which was achieved using Sodium Metatungstate (SMT) monohydrate (3Na2٠WO4٠9WO3٠H2O). SMT, which is friendly to use, health and safety-oriented, showed high recyclability and consistent performance when filtered through membrane filter (millipore 0.45). Magnetic separation was not applied as it was not effective on concentrates. The average recovery of kyanite is 8.5%, but with significant fluctuation (2.1% - 18.7%) depending on the area of study.
Furthermore, 214 microprobe spot analysis and 334 backscattered images were produced through SEM-EDS, concerning granules of kyanite concentrates, to examine the effectiveness of the liberation size and beneficiation method. Most of the granules examined were fragments of pure massive kyanite crystals without mineral inclusions and/or agglomerates with other minerals. Exceptions occurred mainly in the concentrates from Moni possibly due to more frequent appearance of mineral inclusions and to smaller average size of kyanite crystals. It was observed granules of “sandwich-like” agglomerates, mainly consisting of biotite and kyanite intercalations. Less idiomorphic agglomerates of kyanite/almandine, kyanite/apatite and kyanite/goethite have been noticed. Concentrates of smaller granulometry (-212 μm, +150 μm) were also recovered for comparative reasons. The smaller fraction showed significant losses in the total weight of the sample (3% and 4% for Moni and Sifones, respectively) and consequently smaller recoveries, possibly due to incompatible experiment setup and equipment, or to preferential mineral distribution between fractions during sieving. Nevertheless lesser impurities have been observed due to the absence of barren mineral agglomerates. The quality characterization of kyanite concentrates and kyanite agglomerate from Apollonas, concerning their content of kyanite and chemical composition was accomplished by XRD/Rietveld and X-Ray Fluorescence (XRF) analysis, respectively. The kyanite content is relatively high for the concentrates which originate from Skado (81.5%) and Sifones (78.7%) and also for the vein kyanite of Apollonas (76.7%). The content of Moni’s concentrate is the lowest one with 65.2% (wt. kyanite). The efficiency of liberation size and the performance of the beneficiation method were evaluated as “adequate”, according to the results of XRF assays for the concentrates of Sifones and Skado. Their percentages of Al2O3 (57.04%, 57.14%), Fe2O3 (1.19%, 0.92%), TiO2 (0.10%, 0.08%), NaO2+KaO2 (0.20%, 0.33%), meet the industrial criteria for applications in refractory materials (XRF results for Skado and Sifones, respectively). The vein agglomerate kyanite assay is barely beyond the refractory standards, being only 0.11% under the limit for Al2O3 (54.5%), 0.08% over the limit for Fe2O3 (1.5%) and 0.85% over the NaO2+KO2 limit (0.5%). The concentrate of Moni falls out of the limits with 52.35% Al2O3 and 4.10% Fe2O3.
Finally, we tested the ability of kyanite, of the concentrates and of the vein samples from Apollonas, to retain its crystallinity under high temperatures (1,300oC), near the limit of mullitisation (1,350 oC), for a significant amount of time (5 hours). After the first stage (1,300 oC) of calcination, the samples were examined through XRD and we verified that Naxos’ kyanite can endure a high-temperature environment and retain its crystallic structure, almost intact, with only minor traces of mullite noticed, indicating the proximity of kyanite’s mullitisation temperature limit. At the second stage (1,450 oC) of calcination, samples of the first stage were calcinated again for 5 hours, aiming at the complete thermal decomposition of kyanite. XRD/Rietveld analysis followed to identify the mineralogical transformation and quantify the products of the intended mullitisation of the samples. The schist-hosted kyanite concentrates, and vein sample of Apollonas have been converted to aggregates of mullite and silica and all the samples contained high amounts of mullite (> 90%).
The results of the experimental and analytical tests indicate that the exploitation of Naxo’s kyanite occurrences, for refractory purposes, is highly plausible. Hence, we suggest that a semi-industrial beneficiation test should be conducted, incorporating state of the art practices. Obtaining realistic recovery and grade data, the profitability of the potential ore mining can be estimated with a high level of certainty.
