Assessing the performance of a novel pneumatic magnetic separator for the beneficiation of magnetite ore

Highlights

• Dry beneficiation of magnetite ore using pneumatic planar magnetic separator (PMS).

• PMS performance was compared with that of wet magnetite beneficiation technologies.

• The outcomes showed PMS efficiency compared favourably with that of DTR.

• PMS showed superior performance than that of wet drum magnetic separator.

• The benefits of PMS as a unit operation in magnetite concentrators were discussed.

Abstract

A novel low-intensity pneumatic planar magnetic separator designed to recover and concentrate fine-grained magnetite minerals is investigated and the performance compared with those of conventional wet magnetic separators, i.e., Davis tube tester and drum magnetic separator. The results of the studies show that the magnetite recoveries and grades achieved by the novel planar magnetic separator compare favourably with that of the Davis tube tester. Moreover, the outcomes of the studies show that the magnetite concentrates recovered by the planar magnetic separator showed higher magnetite grades with fewer impurities than that of the wet drum magnetic separator. The findings from the study show that the planar magnetic separator could be a potential laboratory-scale substitute for the Davis tube tester, as well as replace wet magnetite beneficiation technologies in drought-ridden and remote regions where the paucity of water could be a potential economic challenge to wet processes.

Introduction

Steel has been a major contributor to the recent global industrialisation due to its unique combination of strength, formability, versatility, recyclability, and low cost (Holmes and Lu, 2015). The primary raw material for steelmaking is metallic iron (Fe) extracted from iron ore. Currently, global steel production is predominantly supported by iron ores sourced from high-grade hematite, goethite, and magnetite (Fe₃O₄) deposits (McNab et al., 2009). Among these sources of iron, hematite resources have dominated iron production because they can be readily processed and upgraded into saleable concentrates through crushing, grinding or milling, concentration by screening, and/or through hydraulic or magnetic classification (Yellishetty et al., 2012). Although magnetite contains the highest iron content (72.4%) compared with hematite and goethite, the majority of the magnetite deposits, especially those located in Australia, contain low iron (~14% to ~45% Fe) and are of low economic value in its low-grade ore state. Invariably, low-grade magnetite ores processing warrants further beneficiation, after crushing and screening, to a higher iron grade concentrate with acceptable impurity levels (Xiong et al., 2015).

Currently, industrial magnetite beneficiation is carried out as a wet process (Karmazin et al., 2002). The most common technology used for magnetite beneficiation is wet drum low-intensity magnetic separators (WLIMS) (Xiong et al., 2015, Dworzanowski, 2010). Conventional WLIMS flowsheets for magnetite beneficiation include multiple stages of LIMS, a combination of WLIMS and magnetic column, or using a combination of WLIMS and reverse flotation (Xiong et al., 2015, Niiranen, 2017). The flowsheet used is usually dependent on the characteristics of the ore. A significant benefit of WLIMS is that it can treat large tonnage of ore (up to 500 t/h) per unit at a minimal operating cost (Xiong et al., 2015).

The current mining climate has seen more magnetite projects being developed in remote and arid regions. For example, South Australia is the driest state in Australia but houses about 14 billion tonnes of low- to medium-grade magnetite deposits (Department of State Development of The Government of South Australia, 2017) that must be developed to obtain their full economic value. The scarcity of potable water for mineral projects in these arid regions can impede the economic application of WLIMS in such projects. This is because the development of mineral deposits located in these arid regions usually requires an extensive water pumping infrastructure such as desalination plant (costing about $0.5 to $1 per m³ of process waster) (Ghaffour et al., 2013) to provide potable water for these projects. To address this challenge, dry beneficiation of magnetite ores offers an opportunity to develop resources in arid regions that would otherwise have not been considered economically viable.

The application of dry processing technologies in iron ore beneficiation is a topical area of research. Song et al. (2019) investigated the use of a fluidised dry magnetic separator to purify and recover magnetite powder from a mixture of magnetite powder and coal powder and concluded that the fluidised dry magnetic separator can improve the magnetite grade and obtain high purification efficiency. In another study, Ezhov and Shvaljov (2015) used a laboratory-scale EVS-10/5 magnetic separator for dry beneficiation of iron ore of the Backer deposit. This was done by analysing the influence of the current strength in the electromagnet winding of the magnetic separator on concentrate yield and recovery. The authors concluded that increasing the current strength increased concentrate yields and at a current strength of 7 A, they obtained the highest concentrate yield of 72.8% with the highest iron content of 48.2%. Dwari et al. (2013) used a combination of dry and wet magnetic separation methods to upgrade a low-grade siliceous iron ore with magnetite, hematite, and goethite as major iron minerals. The study showed that for particles finer than 200 µm, the separation methods produced a magnetic concentrate with 67% iron and iron recovery of 90%. Also, investigations into the use of dry high-intensity magnetic separation for concentrating various paramagnetic iron minerals have been presented by Tripathy et al., 2014, Zhang et al., 2015, Tripathy and Suresh, 2017, and Tripathy et al. (2017b), whilst a review of the present status and prospects of dry high-intensity magnetic separation has been presented by Tripathy et al. (2017a). Kelsey et al. (2017) used the Cyclomag-100 dry magnetic separator to recover and concentrate a finely ground (~100 µm) magnetite ore containing 31.5% Fe and showed that the Cyclomag-100 generated magnetite concentrate with a mass yield of 56%, Fe recovery of 83.5%, and concentrate grade of 46.2% Fe.

Dry low-intensity magnetic separators are more commonly used for tramp iron removal (Elder and Sherrell, 2011) and coarse cobbing of magnetite ores (Aubrey, 1958, Connelly and Yan, 2009). They are generally inefficient for processing low-grade fine-grained magnetite ores (Xiong et al., 2015, Elder and Sherrell, 2011). This is because conventional dry low-intensity magnetic separators are known to yield only appreciable separation when processing feeds with particle size coarser than 75 µm, which are spread in a monolayer over the separator (Svoboda, 2004b). However, low-grade fine-grained magnetite ores generally require fine milling (usually < 45 µm) of its run-of-mine (ROM) ore to achieve enough liberation and as such, this renders the conventional dry low-intensity magnetic separators ineffective. Moreover, spreading fine-milled feed particles in a monolayer over a magnetic separator has a major dust pollution issue.

The challenges related to dry processing of fine-grind magnetite particles motivated the development of an innovative pneumatic planar magnetic separator (PMS). The design and operation of the PMS eliminate dust pollution. Some optimisation and comparative studies conducted using the PMS on different magnetite ores have been presented by Baawuah et al., 2018, Kelsey et al., 2017, and Kelsey et al. (2018). Specifically, Kelsey et al. (2018) highlighted the significance of simplified dry processing flowsheets for magnetite ores using a novel superfine crusher and a planar magnetic separator which offer low-cost dry processing alternative to conventional wet milling and wet magnetic separation.

In this study, we investigate and compare the performance of the novel pneumatic planar magnetic separator with selected conventional wet magnetic separators, specifically, drum magnetic separator (DMS) and Davis tube recovery (DTR) tester. Before this, the study investigated how selected operational parameters (air velocity and magnet disc speed) and feed grind sizes of the PMS affect its performance. The key research questions to be addressed in this study are:

• What operational conditions (air velocity and magnet disc speed) of the PMS will result in its optimum performance in terms of concentrate mass yield, grade, and purity?

• What is the effect of magnetite feed particle sizes on the performance of the PMS?

• How does the PMS performance compare with those of DTR and DMS?

The PMS, pictured in Fig. 1, is made up of a nonmagnetic housing with a circular internal separation chamber, a feed inlet, and concentrate and tailings outlet ports. The separation chamber is sandwiched between rotating discs with embedded permanent magnets alternating with blank disengagement sectors. An internal partition separates the inlet and outlet ports. During operation, the dry feed is transported by pressurised air at a pre-set velocity through the separation chamber, where magnetic particles are recovered and separated from the nonmagnetic particles.

The pneumatic conveyance system of the PMS consists of a prime mover, the feeding system, the mixing and acceleration zone, the conveying zone, and the gas-solids separation zone.

The prime mover supplies the conveying air for transporting the dry feed into the separation chamber. The PMS is equipped with positive and negative pressure pumps at the feed entry and tailing discharge points, respectively, to ensure there is no pressure drop within the separation chamber.

This is the section where the dry feed is introduced into the moving air stream resulting in a substantial change in its momentum. The acceleration zone consists of the horizontal pipe through which the conveying air flows. A venturi allows the mixing of the feed and air and accelerates the feed-air mixture into the separation chamber.

Once the feed has passed through the acceleration zone, it is conveyed through the separation chamber where magnetic force, eddy current, and centrifugal forces separate and recover the magnetic particles from the nonmagnetic particles.

In this zone, a cyclone dust collector is used to separate and recover the tailing particles from the transporting air stream.

The PMS’ inlet air velocity and magnet disc speed have been identified as its fundamental operational parameters that affect its performance. The derivation of these parameters has been summarised in Appendix A for reference.