Fe2O3 hematite quantity increase in quartz sand by heat treatments

Abstract

Heat treatments were performed on the quartz sand to increase the quantity of Fe2O3 hematite phase. The heat treatments were performed on the as-received sand samples. The heating temperatures were chosen in the range of 120-600 C and the time durations in the range of 1-24 h. The sand phases evolution on the temperature was followed by differential scanning calorimetry (DSC). Identification of the phases was realized by X-ray diffraction. The modifications of the iron quantity and distribution in the sand particles were identified by Energy Dispersive X-ray Spectroscopy (EDX) analyses. An optimum temperature/time for the annealing was identified, leading to highest Fe2O3 content. Testes for magnetic separation were performed to validate the method. Introduction At present, there is a steady increase in demand for high purity quartz worldwide [1]. Quartz is used frequently in glass, ceramic and even in nano-industries [2]. Quartz sand is the most common type of sand in the nature [3]. It is used all over the world in different applications because of distinct physical characteristics, like hardness, chemical and heat resistance, also low cost [4]. Depending on the training mode and where it is found, it appears in different shapes and colors [1]. The silicon dioxide that is used to manufacture glass is extracted almost all from quartz sand, which must have over 97 % SiO2 [5]. Usually, the quartz is colorless or white, but the presence of the impurities can change the color. The iron oxide – hematite phase (Fe2O3) is one of the most frequent impurity and depending of the composition concentration, the quartz can alter the color up to yellow [3]. The quality of the sand is as better as the quantity of the iron oxide is smaller. Despite the importance of the sand, the utilization is limited by the quality of the material which contains harmful mineral inclusions. The presence of the impurities, especially iron oxide, limit the sand utilization for high quality glass manufacturing [5]. A big part of the impurities released can be reduced or eliminated by physical operations, such as size separation, spiral concentration, magnetic separation, etc. [6]. The iron oxide from the sand can be reduced also by physicochemical method [4]. The most ecological method to improve the quality of the sand is the magnetic separation method. The magnetic separation is used to decrease and stabilize the iron content [7]. If the method is not effective enough, efficiency can be increased by a thermic treatment, mechanical milling or a specific granulometric class removal. The experiments presented in reference [5], shows that magnetic separation method removes about 80,49 % of iron oxide from sand and decrease the Fe2O3 content from 0,41 % down to 0,08%. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 105-114 doi: http://dx.doi.org/10.21741/9781945291999-12 106 A big part of the impurities presents in the quartz sand contain iron and they are finely dispersed and low magnetic. The special magnetic separators, characterized by high magnetic induction (> 0,6 T), are used to eliminate such impurities. Lately, there where fundamental changes on the separators, especially at the level of magnetic parts. The old magnetic separator systems where replaced by systems that are based on permanent magnets of iron-neodymiumboron type. Thus, this change improved the quantity of the sand and decreased the manufacture cost [8]. The present study is focused on the quartz sand evaluation regarding the iron content and its influence on the color and Fe2O3 phase content. Changes induced by annealing are considered and their effects on the Fe2O3 phase presence. Finally, a basic magnetic experiment is performed for evaluation of the proposed method efficiency. Materials and Methods The quartz sand (Cluj County, Romania) was used for all the experiments. Samples of quartz sands were heat treated at different annealing times from 1h up to 24h. For each test the required amount of samples were placed in a ceramic crucible heated in the furnace. Maximum temperatures of 120, 200, 300, 400, 500 and 600°C were considered. For the thermal treatments was used a programmable INDUSTRY furnace, in oxygen atmosphere. The structural evolution of the samples was highlighted by X-ray diffraction (XRD) using the Cobalt Kα radiation (1.79026 Å) in an Inel Equinox 3000 powder diffractometer in the 2theta = 20-110° range. The occurring of transformation during the heating process were investigated by differential calorimetrical analysis (DSC) using Setaram Labsys equipment. The heating rate was 10 °C/min and the used atmosphere was argon gas. The morphology of the samples was investigated by the JEOL-JSM 5600 LV scanning electronic microscope (SEM) equipped with an EDX spectrometer (Oxford Instruments INCA 200 software). The optical images were recorded using the optical microscope VisiScope TL384M (VWR) type at 40x magnification. The magnetic separation experiments were performed with a commercial NdFeB magnet. Results and Discussions The first effect of sand annealing was the sand colour change. The color of annealed samples for 24 h at various temperatures ranging from 120 °C up to 600 °C is presented in Fig. 1. Fig.1. Color change observed in silica sand grains after heat treatment. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 105-114 doi: http://dx.doi.org/10.21741/9781945291999-12 107 Fig. 1 shows a color change of the sand with the increase of the temperature from yellow to pink-orange. Similar color change upon annealing was observed in [9]. Color change with temperature rise suggests that changes occur in the crystalline structure or impurity phase of sand beyond loss of pores moisture content and dehydration of iron deposits [9]. At temperatures above 250-300 °C, the color changes correspond to the dehydration of the iron compounds as indicated in [10]. The samples observed by eye, were analyzed by optical microscopy, and the recorded images at a magnification of 40X are presented in Fig. 2 for the samples annealed for 24h at different temperatures and in Fig. 3 for samples annealed at 600 °C at different durations. Un annealed TT 120Co/24h TT 200Co/24h TT 300Co/24h TT 400Co/24h TT 600Co/24h Fig. 2. Color change of grain sand with temperature rise during heat treatment (24h annealing time). Optical microscopy images, x40. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 105-114 doi: http://dx.doi.org/10.21741/9781945291999-12 108 In the images presented in Figs. 2 and 3, it is observed a change in the color of the sand grains upon increasing temperature and annealing time. For samples treated at 600 ̊C is observed the most intense pink-orange color. The change of sand color to red suggests the formation of Fe2O3 in sand from additional iron phases. X-ray diffraction (Fig.4) confirm the appearance of the Fe2O3 phase. The color change is observed for samples heated more than 300 °C. For temperatures smaller than 300 °C no color change is recorded. Later in our discussion the occurrence of this color change for high temperature relates to calorimetric measurements and some structural changes upon heating. Un annealed TT 600Co/3h TT 600Co/6h TT 600Co/12h Fig. 3. Change in the color of sand grains depending on the annealing time at TT of 600 ̊C.