Document Type : Research Article
Authors
1 Ferdowsi University of Mashhad
2 ferdowsi university of mashhad
Abstract
Introduction: Sepiolite is a fibrous clay mineral which is usually found in Tertiary sediments in arid environments. The most abundance of sepiolite is between 30⁰ to 40⁰ latitudes of both northern and southern hemispheres. Sepiolite is an Mg-rich clay minerals which is very sensitive to weathering. Sepiolite is an industrial mineral with a variety of applications due to its structural and chemical properties. However, the clay (e.g. palygorskite and smectite) and no-clay (e.g. dolomite and quartz) impurities reduce the quality of sepiolite. Therefore, removing the impurities enhances the quality of the main clay mineral. Mineral purification consists of a series of chemical (e.g. acid treatment) and physical (e.g. particle size fractionation, sieving, ultrasonic treatment) procedures. There is a sepiolite mine in the northeastern Iran, near the city of Fariman. The sepiolite is a sensitive clay to weathering, especially in acidic solution. Therefore, the objective of this study was to propose a simple physical method based on particle size fractionation to purify the sepiolite.
Material and Methods: Sepiolite mine is located around Elyator, a village near the city of Fariman. The relatively hard sepiolite samples were grinded and passed through a 2 mm sieve. To determine the mineralogical composition, the powdered samples were analyzed by X-ray diffractometer (model: Explorer). XRF spectroscopy (model: PHILIPS-PW148) was used to identify the elemental composition. Pipette method was used to separate the particle size fractions. Firstly, the samples were passed through a 270 mesh (50 µm) sieve. The 0-50 µm fraction was then transferred to the cylinder containing dispersion solution (0.1% sodium carbonate and sodium hexametaphosphate solution). Based on the settling time of the particles in the suspension, three classes of particle size of 20-50, 0-20 and 50 µm size (sand size) were about 20 % and those with less than 2 µm size (clay size) consisted 37 % of the sample. Silt size particles (2-50 µm) were about 43 % of the sample. The XRD diffractograms indicated that particle size fractionation considerably decreased the amount of quartz. Dolomite peaks were completely absent in the diffractogram of the < 2 µm fraction. Furthermore, the peaks of palygorskite were not present in diffractogram of 20-50 µm. The intensity of sepiolite peaks considerably increased and the intensities of the other minerals decreased in relation to bulk samples. This confirmed that the most impurities were in the fraction > 50 µm. The ratios of the sepiolite indicator peak to the dolomite, palygorskite and quartz indicator peaks in bulk sample were 5.11, 7.28 and 2.82. This ratio was very high for dolomite in < 2 µm fraction and for palygorskite in 20-50 µm fraction. A purification procedure should be both efficient and economic. The 0-20 µm fraction composed about 70 % the particles. The separation time for this fraction is also pretty fast. Therefore, 0-20 µm particles seem to be economically purified. Based on the conventional measurement method for carbonates (HCl digestion and NaOH titration method), the calcium carbonate equivalent in < 2 µm fraction was calculated to be about 10 % despite removal of dolomite in this fraction. This illustrates that HCl dissolved the sepiolite. However, if removal of dolomite from coarser fraction by HCl is needed, it should be applied in the solution with high amount of Mg to prevent sepiolite dissolution.
Conclusion: Dolomite, palygorskite and quartz were the impurities in Fariman sepiolite. There is no chemical treatment to remove the quartz and palygorskite. Dolomite can be easily removed using HCl, but it dissociates the sepiolite, too. The result indicated that particle size fractionation as a simple physical method purifies sepiolite effectively.
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