International Journal of Mineral Processing 130 (2014) 108–113

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International Journal of Mineral Processing

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Effect of nonmetallic additives on iron grain grindability

Wen-tao Hu a, Hua-jun Wang a,⁎, Xin-wei Liu a, Chuan-yao Sun b

a State Key Laboratory of High-Efficient Mining and Safety of Metal Mines (USTB), Ministry of Education, University of Science and Technology Beijing, Beijing 100083, Chinab State Key Laboratory of Mineral Processing Science and Technology, Beijing General Research Institute of Mining and Metallurgy, Beijing 100070, China

⁎ Corresponding author. Tel.:/fax: +86 10 62332902.E-mail address: wanghuajun@bjjzq.com (H. Wang).

http://dx.doi.org/10.1016/j.minpro.2014.05.0100301-7516/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 October 2013Received in revised form 25 May 2014Accepted 30 May 2014Available online 14 June 2014

Keywords:Ferric bauxiteGrindabilityDirect reduction

This paper investigates the effects of nonmetallic additives on the grindability of iron grains using SEM–EDS andXRD.With different kinds of nonmetallic additives added to direct reduction systems, significant differenceswerenoted in terms of particle size, morphology, and the relationship with slag of iron grains. With the addition ofCaO + Na2CO3, CaCO3 + Na2CO3 or without an additive, high iron grain grindability cannot be achieved.However, when Na2CO3 is used, iron grindability is improved. Under optimal conditions, the grade of ironpowder is 95.6%, and the recovery is 90.2%.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ferric bauxite is one of the most valuable refractory bauxites and iswidely found in China (Deng et al., 2010; Liu et al., 2013; Zhang et al.,2013), Laos (Cheng et al., 2008), and Tanzania (Mutakyahwa et al.,2003). However, ferric bauxite hasn't yet been exploited. The possibleutilization approaches are to extract alumina before iron (Chen andZhou, 1997; Jiang et al., 2000), to extract iron before alumina(Zhu et al., 2009; Shi et al., 2011), beneficiation before Bayer process(Kahn et al., 2003), and biological treatment (Papassiopi et al., 2010;Natarauan, 1997).

Direct reduction has recently become a new research area in relationto ferric bauxite (Hu et al., 2012a, 2012b, 2012c; Yeh and Zhang, 2013;Hu et al., 2013). With this technology, diaspore and boehmite can beconverted to sodium aluminate, which can be extracted with water.Hematite can be reduced to iron and then separated into a magneticconcentrate. Unlike a rich bauxite ore, several nonmetallic minerals,including gangue and nonmetallic additives, are contained in a ferricbauxite reaction system. These nonmetallic minerals directly influencethe iron grain formation and indirectly affect the iron grain grindability.Iron grains are difficult-to-grind, thus it is necessary to select optimumadditive to improve its grindability. The grindability of bauxite ore hasbeen investigated (Mucsi et al., 2011), but previous research on ferricbauxite direct reduction technology mainly focused on optimizationand additive selection (Hu et al., 2012a, 2012b, 2012c; Hu et al., 2013),and did not consider the effect of nonmetallic minerals on iron graingrindability. In this paper, the morphology and paragenesis of irongrains with other minerals were observed using SEM, the chemical

composition of micro zones was observed using EDS, and iron graingrindability was measured by calculating the unit volume productivityof rod mills to reveal the link between nonmetallic additives and irongrain grindability.

2. Experiment

The ore used in this study was supplied by Guangxi Zhuang Autono-mous Region of China. Themulti-element analysis for the ore is given inTable 1, the XRD pattern is shown in Fig. 1, and the SEM–EDS images areshown in Fig. 2.

The results indicate that less than 60% of alumina in the ore exists inthe form of diaspore or boehmite, with the rest existing as kaolinite. Asshown in Fig. 2, minerals in ore have complex associations and dissem-ination, as seen in the microgranular ferric oxide mineral contained indiaspore and parts of alumina found in hematite. The proximate analy-sis and ash multi-element analysis of the coal used in the experimentare shown in Tables 2 and 3, respectively.

The instruments used in the experiment are listed in Table 4. In eachexperiment, 20 g ferric bauxite was subjected to carbothermal reduc-tion with a certain amount of coal and nonmetallic additives in a graph-ite crucible in a muffle furnace. The solids were ground after cooling.

The chemical composition was analyzed using an atomic absorptionspectrophotometer (AAS; Rayleigh, China). The ore mineralogical com-position was determined through XRD analysis (Rigaku, D/MAX-rA,Japan). Themorphology andmicro-chemical composition of the samplewere examined using SEM (ZEISS, EVO 18, Germany) and energydispersive spectrometer (EDS). The functions of nonmetallic additiveson iron grain formationwere investigated by observing themorphologyand chemical composition of iron grains and inclusions.

Table 1The multi-element analysis result of the ore.

Components MgO CaO Na2O K2O TiO2 P2O5 Fe2O3 SiO2 Al2O3 LOI

Content/% 0.68 0.63 0.32 0.06 1.49 0.04 41.13 12.22 33.02 8.97

109W. Hu et al. / International Journal of Mineral Processing 130 (2014) 108–113

The recovery of iron powder, which is the mineral iron concentratein the form of elementary iron, was computed as follows:

εFe ¼ 100 � βα� γ ð1Þ

where:

εFe recovery of iron powder, %;α Fe grade of ferric bauxite raw ore, %;β Fe grade of magnetic concentrate, %; andγ yield of magnetic concentrate, %.

The grindability of iron grain was computed as:

q−74 ¼ 60m � γ−74

100 � V � tp ð2Þ

where:

q-74 productivity of rod mill (yield of −74 μm production),kg·(L·h)−1;

m quantity of the sample, kg;V effective volume of rod mill, L;t the time necessary for grinding, min; andp a non-dimensional number in (0, 1).

3. Effect of nonmetallic additives on iron grain morphology

The morphology of iron grains formed during the reduction processwas studied for three additives: CaO–Na2CO3 mixture, Na2CO3, andNa2CO3–CaCO3 at a fixed amount and reduction temperature. A blankexperiment without additives was also conducted for comparison.

0 20 40 60 80 1000

200

400

600

800

1000

1200

Goethite

Boehmite

Diaspore

Kaolinite

2 Thera (°)

Hematite

Inte

nsi

ty (

CP

S)

Fig. 1. XRD pattern of ferric bauxite ore.

3.1. Blank experiment (without additive)

Themorphology of the iron grains in the reduction product obtainedusing 20% coal at 1250 °C for 90 min is shown in Fig. 3.

The results show thatmost of the iron grains are smaller than 10 μm.Although the reduction temperature reached 1250 °C, seldom micro-fine iron grains sintered. The results also show that Si and Al elementsin the reduction product are present in the form of silicate and corun-dum. These minerals easily become inclusions, and reduce the overallquality of the product.

3.2. Effect of CaO–Na2CO3 mixture on iron grain morphology

Themorphologies of iron grainswith 18.13%Na2CO3 and 28.57%CaOat 1150 °C for 45 min with 20% coal are shown in Fig. 4.

The iron grains are comparatively coarser than grains obtainedwithout addition. Moreover, most grains are sintered. Si and Al in thereduction product exist mainly as silicate, in which this mineral easilybecomes inclusions, and reduces the overall quality of magneticconcentrate.

3.3. Effect of Na2CO3 on iron grain morphology

Themorphologies of iron grains obtained at 1150 °C for 45minwith20% coal and 28.57% Na2CO3 are shown in Fig. 5.

Iron grains are coarser than those obtained with former additions,which implies that iron grains sintered. Thus, a high degree of mineralmonomer separation will be more easily achieved without ultrafinegrinding. Si and Al in the reduction product mainly exist as aluminosil-icate. However, with a coarse iron grain size and high degree of mineralmonomer separation, the inclusion content in the iron powder can bemore easily controlled.

3.4. Effect of Na2CO3 and CaCO3 mixture on iron grain morphology

Fig. 6 shows the morphologies of iron grains gained at 1150 °C for45 min with 20% coal, 9.02% CaCO3, and 19.55% Na2CO3.

As shown in Fig. 6, most iron grains are clastic, which indicates thedifficulty in obtaining an extremely high degree of mineral monomer.However, most inclusions in the reduction product existed as CaCO3,which is a useful composition. Thus, the Na2CO3 + CaCO3 mixture didnot have an adverse effect on iron powder quality.

The results show that nonmetallic additives take a significant effecton iron grain morphology. The optimum grains with satisfactorysize and morphology are obtained with Na2CO3 as the non-metallicadditive.

4. Effect of nonmetallic additives on iron grain grindability

With the three nonmetallic additives, the particle size, morphology,and relationship of iron grains with slag appear significantly different.These differences suggest that the nonmetallic additive would impactiron grain grindability. Grindability is measured by calculating the unitvolume productivity of the rod mill. The grinding curve of directreduction product is presented in Fig. 7.

According to Fig. 7, the rod mill unit volume productivity of irongrains can be calculated when p = 0.5, the median of (0, 1), and theresults are presented in Table 5. Table 5 shows that thedifferent nonme-tallic additives have a significant effect on the rodmill unit volume pro-ductivity. For Na2CO3, the productivity was 0.28 kg·(L·h)−1, whereasfor CaO + Na2CO3 the productivity was only 0.18 kg·(L·h)−1. Theiron grade (TFe) and iron recovery (εFe) of iron powder with a grindingfineness of−74 μmat 85% are presented in Table 6. The results indicatethat nonmetallic additives have a significant effect on the iron particlesizes, grade, and recovery.

(a)

(d)

(b) (c)

(e)

(f) (g)

Fig. 2. SEM photo of ferric bauxite ore (a), EDS pattern of point 1 (b), EDS pattern of point 2 (c), EDS pattern of point 3 (d), EDS pattern of point 4 (e), EDS pattern of point 5 (f), and EDSpattern of point 6 (g).

110 W. Hu et al. / International Journal of Mineral Processing 130 (2014) 108–113

5. Conclusions

The effects of nonmetallic additives on iron grain grindabilitywere investigated using XRD and SEM–EDS. The optimal iron grain

Table 2Proximate analysis of coal.

Air-driedmoisture/%

Air-driedvolatiles/%

Air-driedash/%

Air-dried fixedcarbon/%

9.16 39.42% 5.07% 46.35%

grindability, rod mill unit volume productivity of 0.28 kg·(L·h)−1,was obtained with the additive of Na2CO3. At the grinding fineness of−74 μmat 85%,most iron grainswere fine-granular and achievemono-mer separation. Under optimal conditions, the grade and recovery ofiron powder were 95.67% and 90.22% respectively. Instead, withoutnonmetallic additives, the size of iron grains was ultrafine, making rodmill unit volume productivity at only 0.22 kg·(L·h)−1.

With the additive of CaO + Na2CO3, seldom iron grainsachieve monomer separation under the grinding fineness of−74 μm at 85%. The unit volume productivity of rod mill was0.18 kg·(L·h)−1.

Table 4Experimental instrument list.

Instruments Rod mills Muffle Mixer Balance Filter Magnetic tube Drying oven

Model XMB-70 SX2-10-13 KJ-1 AR1140 XTLZ CXG-99 PH050

Table 3The multi-element analysis of the coal ash.

Components SiO2/% Fe2O3/% Al2O3/% CaO/% MgO/% K2O/% TiO2/% Na2O/% P2O5/%

Content/% 38.0 22.57 21.37 7.15 1.9 1.38 0.84 0.43 0.41

111W. Hu et al. / International Journal of Mineral Processing 130 (2014) 108–113

The size of iron grains obtained with CaO + Na2CO3 wasgreater than that obtained without additives, but there weremore inclusions in them. This suggests that ultrafine grinding isnecessary to make these iron grains achieve monomer separa-tion. So, it is very difficult to obtain high grade iron powderwith CaO + Na2CO3.

(a)

(b)

(d)

Fig. 3. Iron grains gained at 1150 °C for 45 min without addition (a), EDS pattern of point 1 (

With CaCO3 + Na2CO3, the iron grains obtained were fine-granularand have someharmless inclusions in them. The unit volumeproductiv-ity of rodmill was 0.20 kg·(L·h)−1. At a grinding fineness of−74 μmat85%, most iron grains achieve monomer separation. Under the optimalconditions, the grade and recovery of iron powder were 92.16% and91.27% respectively.

(c)

(e)

b), EDS pattern of point 2 (c), EDS pattern of point 3 (d), and EDS pattern of point 4 (e).

(a)

(b)

(c)

Fig. 5. SEM photo of iron grains gained at 1150 °C for 45 min with 20% coal (a), 28.57%Na2CO3, EDS pattern of point 1 (b), and EDS pattern of point 2 (c).

(a) (b) (c)

Fig. 4. SEM photo iron grains gained at 1150 °C for 45 min with 28.57% CaO (a), EDS pattern of point 1 (b), and EDS pattern of point 2 (c).

112 W. Hu et al. / International Journal of Mineral Processing 130 (2014) 108–113

In conclusion, nonmetallic additives significantly influence irongrain grindability. And the optimum productivity, with satisfactorygrade and recovery of the iron, was obtained with Na2CO3 as the non-metallic additive.

6. Acknowledgments

This study was supported by the National Natural Science Founda-tion of China by a grant number 51304012, the China PostdoctoralScience Foundation by a grant number 2013M530529, and the OpenFoundation of the State Key Laboratory of Advanced Metallurgy(USTB) by a grant number KF 13-05.

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(a)

(b) (c)

Fig. 6. SEM photo of iron grains gained with CaCO3 + Na2CO3 mixture (a), EDS pattern of point 1 (b), and EDS pattern of point 2 (c).

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0 20 40 60 80 100

30

40

50

60

70

80

90

100

Grinding time (%)

No additives

Na2CO

3 and CaO mixture

Na2CO

3

Na2CO

3 and CaCO

3 mixture

Gri

ndin

g f

inen

ess

(%)

Fig. 7. Grinding time — fineness curve.

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Table 5The rod mill unit volume productivity of iron grain.

Nonmetallic additive CaO + Na2CO3 Na2CO3 CaCO3 + Na2CO3

Grinding time/min 48.25 69.79 30.02 57.22Productivity/kg·(L·h)−1 0.22 0.18 0.28 0.20

Table 6The TFe and εFe of iron powder with a grinding fineness of−74 μm at 85%.

Nonmetallic additive CaO + Na2CO3 Na2CO3 CaCO3 + Na2CO3

TFe/% 76.21 67.23 95.67 92.16εFe/% 87.56 98.31 90.22 91.27