Study on the particle size reduction by milling of quartz sand for magnetic separation

Abstract

For being used in crystal glass industry, the iron content of quartz sand must be under 0.09 %. If the reserve contains a higher quantity, methods for iron reduction must be used. Usually the iron phases are present in large quantity in the small particle size fraction. For reducing the sand grain size, milling was performed on a planetary ball mill. Different ball/powders ratio were studied for determining an optimum particle size vs. milling duration. The particle size was determined for each milling experiment. Using Energy Dispersive X-ray spectroscopy (EDX), the elemental distribution for the particle was quantified. By X-ray diffraction, the phase distribution of the sand was analyzed and correlated with the chemical composition. The phases are changing their ratio versus the grain size. The main phase is SiO2 as quartz, accompanied by minor phases: iron oxides (Fe3O4, Fe2O3, and FeTiO3) and some oxide of Al, Na, Ca, and K. Testes for magnetic separation were performed for validating the method. Introduction The quartz sand is the raw material for glass industry. Unfortunately, as all raw materials, quartz sand purity is the limiting criterion for his usage, since the structure and composition give the properties, the usage and classification criteria for glasses [1]. The most detrimental impurity in the quartz sand is iron, followed by some other metallic oxides (titanium, cobalt, copper, etc.). The effect of metallic impurities in the sand is most commonly observed in color of the resulting glass [2]. The minimum iron quantity in the sand for obtaining a color glass is 0.1 %. The classical way for iron removing is flotation, using toxic reagents as amine, NaOH or H3PO4 [3 5]. A cleaner approach is magnetic separation [6]. In magnetic separation experiments, the content in magnetic phase (iron oxides) and particle size represent a key factor for an efficient removing setup [6]. Also, the different types of magnetic separators are considered [7]. A method of controlling the particle size of the sand is by ball milling [8]. In the milling experiments the particle size modification is realized by collision events between balls and sand particles [8]. For our studies is suitable that a high productivity to be achieved, at small milling time and the powder to be produced in a continuous way [9]. For high productivity, the quantity of sand is analyzed and in the milling experiments can be expressed in the form of ball to powder mass ratio (BPR). A high BPR means less quantity of material for processing and small BPR means high material quantity introduced in milling chamber. One purpose of this study is to determine optimal condition of sand milling considering different BPR and milling times. Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 95-104 doi: http://dx.doi.org/10.21741/9781945291999-11 96 The ball milling was found to be useful in sand purification and particle size reduction [10]. In the milled sand it is important to have an insight on the iron phase distribution in the particles [11]. Scanning electron microscopy (SEM) coupled with X-ray energy dispersive spectrometry (EDX) was found to be a suitable technique for sand characterization, as proved by several studies [12 16]. In the present study, the particle size, the iron distribution and iron quantity is studied for the quartz sand ball milled, in order to establish the optimum parameters for magnetic removal. Experimental The particle size analysis was performed on standard sieving method using sieves in the range 40 – 800 μm. The standard sieved sand quantity was 100 g. Multiple sieving were performed to obtain statistical data. Supplementary, particle size distribution analysis were recorded using an Analysette 22 Nano Tec particle analyzer. The size range was from 0.1μm to 135μm. From the particle size distribution, the parameters D10, D50 and D90 were determinate. D10, D 50 and D90 represents the mean powder diameter equal with a diameter more or equal with 10, 50 and 90 % from the total powder volume. The resulting particle size ranges were morphological and compositional characterized by Scanning Electron Microscopy using a JEOL JSM 5600LV microscope equipped with an EDX spectrometer (Oxford Instruments, INCA 200 software). Crystallographic analysis was performed by X-ray diffraction on a INEL 3000 Equinox diffractometer, operating with CoKα radiation (λ = 1.79026 Å) in the angular range 2θ of 20 -110 °. Optical microscopy was performed on a VWR microscope. The milling experiments were performed with a planetary ball mill, Fritch Pulverisette 6. The main disc velocity was 350 rpm, for a hardened steel vial with 100 balls. The milling was conducted in air at several ball to powder weight ratio (BPR) for times up to 10 min. Results and discussion The milling experiments on quartz sand begun with the analysis of the as received sand. As the milling has its major influence on the particle size distribution, the distribution of the particle in the as-received sand is considered firstly, and is presented in figure 1. Fig. 1. Particle size distribution of the as-received sand. The distribution is obtained by sieving. 0 100 200 300 400 500 600 700 800 900 0 5 10 15 20 25 30 35 40 45