Production of pelletizing concentrates from ... Production of pelletizing concentrates from Zandrivierspoort

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  • Introduction

    Kumba Iron Ore is looking to grow its production from the current 44 M/ta to 70 M/ta by 2019. Of this planned growth, approximately 6 Mt/a is scheduled to come from projects in South Africa’s Limpopo Province. The Zandrivierspoort (ZRP) project aims to mine and beneficiate a magnetite resource with low contaminant levels to produce from 1 Mt/a up to 2.5 Mt/a product in the form of either a concentrate, micro-pellets for direct reduction, or blast furnace pellets. Anglo American Technical Solutions Research conducted piloting campaigns with a bulk sample obtained from the ZRP site. Kumba Iron Ore is currently designing a plant that will process the ZRP magnetite-haematite ore- body. Anglo American Technical Solutions Research conducted these piloting campaigns to validate the process design flow sheet, demonstrate process performance, and to generate data for use as design basis. Secondary to this, the pilot plant was to produce approximately 7 t of magnetite-

    haematite concentrate with low enough SiO2 to be used as feedstock for pelletizing studies at Kumba’s Value in Use Laboratory and elsewhere. The piloting campaign entailed crushing the run-of-mine to -1 mm prior to low-intensity magnetic separation (LIMS) and rare earth drum separation (REDS) for haematite scavenging, ball milling in closed circuit with a vibrating screen, cleaning of the ball mill circuit product through three stages of LIMS, IsaMilling the ball mill LIMS concentrate, and further re-cleaning of the IsaMill product through three LIMS stages. A combination of LIMS and REDS in different parts of the circuit was chosen to recover magnetite and liberated haematite respectively. Magnetite is ferromagnetic, thus it can be effectively recovered using LIMS (800–2000 G) while haematite is paramagnetic, thus requires high magnetic field intensities (>6000 G) (Wills and Napier Munn, 2006). This paper presents interpretations of the feed sample characterization, pilot plant mass balance results, and the implications for the proposed ZRP plant flow sheet.

    Background

    ZRP geological outline

    The Zandrivierspoort prospecting right is located approximately 25 km northeast of Polokwane on the farm Zandrivierspoort 851 LS, in South Africa’s Limpopo Province, as shown in Figure 1 (Kumba Iron Ore, 2013). The low-grade magnetite project mineral resource base is estimated at 476.1 Mt with 34.5% Fe (Kumba Iron Ore, 2013). Zandrivierspoort is a banded iron formation

    Production of pelletizing concentrates from Zandrivierspoort magnetite/haematite ore by magnetic separation by P. Muthaphuli*

    Synopsis Kumba Iron Ore’s Zandrivierspoort (ZRP) magnetite-haematite project aims to mine and beneficiate a magnetite resource with low contaminant levels to produce from 1 Mt/a to 2.5 Mt/a product, which will be either a concentrate, micro-pellets for direct reduction, or blast furnace pellets. Anglo American Technical Solutions Research conducted a number of piloting campaigns to validate the process design flow sheet, demonstrate process performance, and to generate data for use as a design basis. Secondary to this, the pilot plant was to produce approximately 7 t of magnetite-haematite concentrate with low enough SiO2 to be used as feedstock for pelletizing studies.

    The pilot plant achieved 34.2% mass yield, at 69.0% Fe, 2.2% SiO2, from an IsaMill grind of 80% -45 µm. This product quality will compare favourably to other leading direct reduction (DR) pelletizing concentrates. However, two-stage comminution to 80% -75 µm and beneficiation of ZRP ore will produce a blast furnace (BF) quality pelletizing concentrate, indicative quality being 67.6% Fe and 4% SiO2, at higher mass yields. An economic evaluation between producing a DR and a BF pelletizing concentrate will have to be conducted prior to finalizing the concentrate production flow sheet..

    Keywords Magnetic separation, iron ore, magnetite, haematite.

    * Anglo American Mining and Technology, Technical Solutions Research.

    © The Southern African Institute of Mining and Metallurgy, 2014. ISSN 2225-6253. This paper was first presented at the, Physical Beneficiation 2013 Conference, 19–21 November 2013, Misty Hills Country Hotel and Conference Centre Cradle of Humankind, Muldersdrift.

    505The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 114 JULY 2014 ▲

  • Production of pelletizing concentrates from Zandrivierspoort magnetite/haematite ore

    (BIF) magnetite deposit in the palaeoproterozoic Rhenosterkoppies greenstone belt or Rhenosterkoppies fragment, which occurs to the northwest of the main, northeast-trending Pietersburg greenstone belt. Figure 2 represents a vertical profile (slice) intersecting the three- dimensional model of the Zandrivierspoort deposit (A-A1 red line in Figure 1), demonstrating the company’s interpretation of the relationship between the orebodies, waste material, and local geology (Kumba Iron Ore, 2013).

    BBulk sample preparation and characteristics

    Kumba Iron Ore delivered approximately 38 t of run-of-mine (ROM) bulk sample at a 150 mm top size. This was reduced using a jaw crusher to 100% passing 70 mm. The jaw crusher product was further crushed to 100% passing 40 mm prior to comminution by high-pressure grinding roll (HPGR). The HPGR crushing was conducted at Mintek using a Koppern HPGR unit to produce 100% passing 1 mm product, which yyields more than 60% liberation of the magnetite and haematite combined. On average, the bulk sample received contained 35.7% Fe with a 40:60 magnetite to haematite ratio. Figure 3 shows the bulk mineralogical composition of the bulk sample processed through the pilot plant.

    Previous company internal data indicated that the average ZRP orebody contains approximately 40.8% magnetite with a 70:30 magnetite to haematite ratio (Kumba Iron Ore, 2013). The implications of the lower magnetite to haematite ratio of this bulk sample on the pilot plant mass yield will be highlighted and discussed under the mass balance results.

    The Bond Ball Mill Work Index (BWI) was determined at a limiting screen of 75 µm. The BWI test was carried out in accordance with the modified Warren Spring Laboratory method. The BWI of this ZRP ore was measured at 17.6 kWh/t, which is in line with previous values obtained from other hardness testing carried out on ZRP drill cores at Anglo Research (van Drunick, 2007). A grind of 80% passing (P80PP ) 75 µm was chosen targeting a blast furnace pelletizing concentrate with 67% Fe and less than 5% SiO2, while a 45 µm grind was targeting a direct reduction pelletizing concentrate with a much cleaner product containing 69% Fe and less than 2% SiO2. Batch grinding and Davis tube tests at 1000 G (Murariu and Svoboda, 2003) had shown that these qualities were achievable at P80PP of 75 µm and 45 µm.

    Piloted flow sheets

    High-SiO2 concentrate production (blast furnace

    pelletizing concentrate)

    Figure 4 illustrates the first phase of the piloting campaign, in which the circuit was operated with a cobbing and a REDS scavenger magnetic separator, the ball mill in closed circuit with a 75 µm screen, and the product of the ball mill circuit reporting to three stages of LIMS cleaning. The ZRP ore consists of magnetite and haematite, a significant amount of

    506 JULY 2014 VOLUME 114 The Journal of The Southern African Institute of Mining and Metallurgy

    in Limpopo Province (Kumba Iron Ore, 2013)

    Figure 2—Southeast-northwest profile depicting the local geology through the Zandrivierspoort magnetite deposit (Kumba Iron Ore, 2013)

    Figure 3—Bulk mineral composition of the ZRP bulk sample used for piloting

    Figure 4—Flow sheet for production of a BF pelletizing concentrate

  • wwhich has been shown to be intimately associated with the magnetite. The product of the REDS scavenger was combined wwith the cobbing product to feed the ball mill circuit.

    The addition of a REDS scavenger improves yield due to the recovery of additional mass of fully liberated haematite wwhich is not recovered across the cobbing LIMS because of its lower magnetic susceptibility.

    Low-SiO2 concentrate production (direct reduction ppelletizing concentrate)

    The blast furnace pelletizing concentrate was sent to the AAnglo American Platinum Division Metallurgical Laboratory (DML) for IsaMilling using a 100 litre (M100 IsaMill) pilot plant unit with 75 kW installed power. Davis tube test work data showed that it was possible to reduce the SiO2 content in the concentrate to around 2% from a grind of 80% -45 µm.

    The IsaMill product was re-pulped using a ball mill to break lumps and allow for dilution prior to the three LIMS cleaning stages as illustrated in Figure 5. This circuit was run at very low feed densities as its primary objective was to produce a high-quality concentrate that is low in SiO2. Thus it was critical to avoid SiO2 entrainment, which would have been worsened by high feed densities.

    EEquipment specification

    Ball mill and screen

    Table I and Table II summarize the ball mill and screen specifications respectively. The ball mill had rubber liners. The screen was a retro-fit of a linear screen that was modified and fitted with a 75 µm woven wire cloth due to the lack of pilot Derrick screens in the country at the time of the test work. The screen deck was re-designed and fitted in- house.

    Magnetic separator specifications

    The four LIMS units were salvaged from an old magnetic separation plant and refurbished in-house with assistance from Eriez Magnetics. The REDS was hired from Multotec. Table III summarizes the magnetic separator specifications and operational settings empl