Finding baseline flotation parameters for the reverse flotation beneficiation of Sishen low grade banded iron ore

Abstract

Anglo American Kumba Iron Ore, a member of the Anglo American plc group, owns high-quality resource bases in both South Africa and Brazil. One of these, Sishen iron ore mine, is a South African mine, where lumpy high-grade hematite ore is treated. As these easily treatable resources depletes, alternative processing methods need to be investigated to ensure sustainability. Low-grade iron ores are commonly beneficiated by a combination of magnetic separation, gravity separation and direct- or reverse flotation. Fine size iron ore fractions, less than 150 micron, requires flotation to be beneficiated to high-grade saleable iron ore product for pelletizing. Three flotation routes have been developed for iron ores outside of Africa: (i) reverse cationic flotation of quartz; (ii) direct anionic flotation of iron oxides; and (iii) reverse anionic flotation of activated quartz. Although vast ores of increasingly complex nature have been identified in Africa, existing literature does not mention any iron ore flotation practices that are currently employed or investigated on this continent. The aim of this study was to prove that low grade Sishen banded iron ore can be upgraded to a saleable iron product by froth flotation. This was proven by a mineralogical study of banded low grade Sishen iron ore; the evaluation of suitable reagent suites; flotation cell conditions; additional beneficiation circuit stages; evaluation of the developed flotation regime on varied feed compositions and determination of correlations between the mineral liberation analysis and flotation results. The fundamental mechanism for selective flotation of hematite is attributed to the selective depression of hematite, where quartz particles may be removed from hematite in a narrow pH range. In this pH range, amine collectors form an ion-molecular complex, which induces hydrophobicity on the surface of quartz particles and attain its frothing character. Reverse cationic flotation is the most-common approach for hematite-quartz separation. Amines are used as collecting reagent; two amine variants supplied by Clariant, have been identified to exhibiting superior silica collecting performance. Starch is the most commonly employed hematite depressant. The ore characterisation of seven clearly identified low grade banded iron formation, BIF, iron ore samples from Sishen iron ore mine, and an equal blend of these seven BIF samples comprises of Chemical composition (by x-ray fluorescence, XRF), mineralogical composition (x-ray diffraction, XRD and mineral liberation analysis, MLA), particle size distribution (sieve tests and laser diffraction) and mineral liberation (by scanning electron microscopy, SEM and MLA). Baseline flotation parameters for reverse flotation of low grade Sishen iron ore BIF, which included bench-scale laboratory flotation tests were established by the evaluation of the depressant type, depressant dosage and depressant dosing stage, dispersant type and dispersant dosage, collector dosage and mixed collector ratio, grinding size and flotation circuit configuration. Results from this investigation identify hematite as the main iron bearing mineral in low grade Sishen iron ore and quartz as the main gangue mineral, with limited amounts of annite and kaolinite. The iron content ranges between 25% and 39% for the received individual samples. The particle size distribution, with 80% passing 1.1 mm, was too coarse for flotation. Grinding curves were constructed to produce a fine flotation, medium flotation and coarse flotation particle size range. Laboratory bench scale flotation tests confirm strong pH dependence for the recovery of hematite, where a natural hydrophobicity is exhibited at pH 7. These tests confirmed that caustisized starch is a suitable depressant for hematite minerals in a low grade Sishen BIF iron ore and that the addition of sodium silicate dispersant may improve the iron product grade. The depressant dosage has a minor effect on the flotation results, and the depressant mechanism is largely dependent on the flotation pulp pH. Froth stability is greatly influenced by a change in the amine collector dosage, where excessive amounts of fine particles also over stabilise the froth bed and it is not possible to control the froth bed height at collector dosages of larger than 25 g/t/stage. Alteration of the flotation circuit, from a six stage rougher float to a nine stage rougher float and six stage scavenger float, after additional milling and depressant dosing, yielded the required target of at least 64% Fe and 30% recovery to the tails(iron product). With no preparation by de-sliming, an iron recovery of 34.7% to the iron product can be achieved by flotation alone. At 66.4% iron, MLA analysis confirmed the iron product to be of a saleable high grade. The MLA analysis also showed the limit of flotation performance to be an excessive entrainment of iron-containing particles after the second scavenger flotation stage. The use of this reagent suite on individual BIF ores shows that more than 98.5% of the silicate materials can be removed to the froth phase. As with the composite BIF material, an additional scavenger stage is required to reach the target percentage iron recovery of 30%.