Microscopy and Microanalysis of Hematite Precipitates from the Zinc Industry

Abstract

In commercial zinc operations, sphalerite (Zn,Fe)S concentrates are roasted in air to produce a ZnOrich calcine; the Fe in the sphalerite is converted mostly to ZnFe 2 O 4 . The calcine is treated in sulphuric acid media to dissolve the Zn, but the associated Fe also dissolves. The solubilized Fe is commonly precipitated as a jarosite-type compound KFe 3 (SO 4 ) 2 (OH) 6 or hematite Fe 2 O 3 [1,2]. In current industrial practice, both the jarosite and hematite residues are sent to disposal sites. Although the jarosite process is more commonly used, jarosite precipitates are voluminous and require an extensive containment area [3]. The hematite process is more costly, but produces a compact, more stable material for disposal; furthermore, hematite precipitates have some market potential [4]. The hematite precipitates commonly contain 0.3-1.0 % Zn, 5-10 % SO 4 and 3-6 % H 2 O, impurities that presently restrict the use of the hematite residues. Consequently, a study was carried out, using multiple analytical techniques, to determine the compositions, microstructures and impurity forms of the hematite precipitates. The overall objective was to enhance the utilization of the hematite precipitates, thereby lessening the environmental impact of iron residue disposal.In addition to the commercial hematite residues, a synthetic hematite precipitate prepared under conditions simulating those of the industrial process, but without the presence of Zn, was also studied. Prior to analysis, all the samples were subjected to extensive water-washing, and were then air dried at 110 o C for 24 h. Various XRD, SEM/EDX and EPMA analyses were carried out; sulphide sulphur was distinguished from sulphate sulphur by X-ray crystal spectrometry on the EPMA. Complementary TGA-DTA-FTIR analyses were carried in a helium atmosphere, and the evolved gases were analyzed.The synthetic hematite precipitate occurs as 1-10 µm spheroids, agglomerates or hollow spheroids. Most of these particles consist of radial crystallites, and have large surface areas, implying a high potential for adsorbing soluble species. The SEM study showed that the sample consists of only hematite; the hematite XRD lines are sharp, implying well crystallized Fe 2 O 3 . Detailed EPMA of the hematite particles gives: 65.2% Fe and 4.7% SO 4 , with 2.3% H 2 O calc . The analysis is identical to the bulk composition of the precipitate. The FTIR analysis shows the separate evolution of H 2 O and SO 2 , implying that the gas evolution reaction is not the decomposition of H 2 SO 4 , as the decomposition of acid would result in the simultaneous release of both gases. The large temperature ranges for H 2 O (150-550 o C) and SO 2 (600-900 o C) evolution suggest that the H 2 O is not loosely adsorbed on the hematite particles and that the SO 4 is not structurally bonded, as loosely adsorbed water would be released at a lower temperature and structurally bonded SO 4 would be evolved over a narrower temperature range. The "water" is likely present as OHwhich replaces O = in the hematite...