The iron ore concentration tailing was provided by the Vale S.A.

The experiments were performed using sulphuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid (HNO₃), citric acid (C₆H₈O₇), oxalic acid ((COOH)₂∙2H₂O), hydrofluoric acid (HF) and sodium hydroxide (NaOH); all of them of analytical grade.

The chemical characterization of the samples and the analytical monitoring of the experiments was done by wavelength dispersive X-ray fluorescence spectrometer―WDX (Rigaku―model ZSX Primus II), energy dispersive X-ray fluorescence spectrometer (SHIMADZU, model EDX-720), flame absorption atomic spectrometry Varian/Agilent (model AA240FS) and the determination of the minerals phases was performed by X-ray diffraction with a Rigaku spectrometer (model D/Max Ultima).

Once dry, the sample was de-agglonerated, homogenized, quarted and packaged. After this preparation, the physical and chemical characterization, particle size analysis, and density measurement were performed.

Based on the granulochemical analysis, the fraction between the sieves of 298 mm and 37 mm was separated, which was subjected to wet and dry magnetic concentrations to remove the magnetic fração, iron-rich part. The magnetic separations experiments were performed in a high intensity, magnetic roller separator (RER4), a medium intensity, drum magnetic separator (WDRE), and a high intensity electromagnetic separator with magnetic field of about 16,000 gauss. Part of these experiments were carried out by Inbras-Eriez.

The leaching experiments, made in batch, were carried out in a beaker with mechanical stirring in water bath and temperature control. In this step the following process variables were investigated: type and dosage of the leaching agent, agitation time, relation between leaching agent and ore, percent of solids, and temperature. The acid digestion experiments were performed in a 150 mL beaker on a hot plate at 200˚C for 4 h and 6 h. The digestion product was leached with water under mechanical stirring at room temperature (25˚C) for 1 h.

The chemical analysis of the sample showed Fe (40.1%), SiO₂ (33.4%), Al₂O₃ (8.31%), P (0.18%), MnO (0.34%) and major mineral quartz (SiO₂), hematite (Fe₂O₃), dickite (Al₂Si₂O₅(OH)₄), and goethite (FeO∙OH). The density was 3.696 g/cm³.

The mass and metallurgical distributions of the main constituents are presented in Table 1. As noted, the fraction + 37 µm recovered 38.6% of the mass and

75.2% of SiO₂. In the granulometric classification between −298 + 37 µm, the silica content increased from 33.4% to 67.0%.

In the dry test, the silica content increased from 67.0% to 95.4% in 3 steps and in the wet test, the silica content increased from 67.0% to 94.5% in 2 steps. In the dry test, 44.8% of mass and 69.7% of silica were recovered, while in the wet test, 55.4% of mass and 84.1% of silica were recovered. The concentration of the original constituents of the sample and the fraction −298 + 37 µm of magnetic and non-magnetic fractions are shown in Table 2.

Initially, tests were performed to choose the best leaching agent for the dissolution of Fe and Al. As mentioned above, the leaching agents selected were the acids HCl, H₂SO₄, HNO₃, HF, C₆H₈O₇, and (COOH)₂∙2H₂O. The experiments were performed at 70˚C, 3 hours of reaction and 10% of solids. The results are shown in Table 3.

As shown in Table 3, under the test conditions, there was no significant difference between the leaching agents investigated in the purification of silica.

Original sample: 0.90% Fe; 3.00% Al₂O₃ and 94.5% SiO₂.

Original sample: 0.90% Fe; 3.00% Al₂O₃ and 94.5% SiO₂.

However, H₂SO₄ was the least efficient, showing the lowest dissolution of Fe.

To remove the Al in the sample, alkaline leaching tests with NaOH were performed. Tests were performed with NaOH 5 mol/L and 12.5 mol/L (~50% w/w), where the percentage of solids, reaction time, and temperature were varied. The experimental conditions and results of these tests are presented in Table 4.

According to the data in Table 4, the best results were obtained when using NaOH solution of 12.5% at 70˚C, with Al dissolution of 46%. The results also showed that in this case a high percentage of solids (50% w/w) can be used.

In a following stage, alkaline leaching residues were subjected to acid leaching to increase the purity of the silica. The leaching agent used was HCl at the concentration of 5 mol/L. Table 5 shows the results and the conditions of these experiments.

The results in Table 5 are inconclusive. However, they show the difficulty of removal of iron and aluminum from the sample after magnetic separation.

Still in search of better conditions for reducing the level of impurities in the residue to obtain high-purity silica, another leaching test was performed with a mixture of HF 50% and HCl 25% in relation 1:1. The acid concentrations and the ratio of their mixture were chosen based on literature (Abdelkrim, 2009). The percentage of solids was 50% with a reaction time of 3 h at room temperature. The residue was washed with distilled water and dried at 110˚C. After that,

Original sample: 0.74% Fe; 2.56% Al₂O₃ and 95.4% SiO₂.

Tabel 7. Fe, Al₂O₃ e SiO₂ grade in residue from acid and alkaline digestion of the non- magnetic fraction.

Original sample: 0.74% Fe; 2.56% Al₂O₃ and 95.4% SiO₂. RAM = acid/ore relation.

a very alkaline leaching with NaOH 12.50 mol/L was performed, aiming at the dissolution of the remaining Al. The results of these tests are presented in Table 6.

As shown in Table 6, for the conditions under which the tests were performed, there was no gain in purification of silica compared with previous experiments. Another expected drawback with the mixture HF/HCl is a high dissolution of silica (7.0% w/v). In leaching experiments, the silica content raises from 94.0% to 98.0%. Thus, the acid digestion with concentrated H₂SO₄ at high temperature was investigated, as shown in the next section.

Acid digestion tests were performed at 200˚C, using concentrated H₂SO₄ with three different sample/acid relations, 500, 1000 and 1500 kg/t, and 4 hours of reaction. The conditions and results of these experiments are presented in Table 7. After completion of the reaction time, distilled H₂O was added at a ratio of 10% of solids and stirred for one hour before proceeding to filtering the pulp.

According to the results in Table 7, in acid digestion tests, higher Fe and Al dissolution was obtained. The silica content was maintained at 98.0%.