Preparation of synthetic rutile from reduced ilmenite through the aeration leaching process

Journal Pre-proof Preparation of synthetic rutile from reduced ilmenite through the aeration leaching process Junyi Xiang, Guishang Pei, Wei Lv, Songli Liu, Xuewei Lv, Guibao Qiu

PII:

S0255-2701(19)30455-6

DOI:

https://doi.org/10.1016/j.cep.2019.107774

Reference:

CEP 107774

To appear in:

Chemical Engineering and Processing - Process Intensification

Received Date:

16 April 2019

Revised Date:

6 August 2019

Accepted Date:

3 December 2019

Please cite this article as: Xiang J, Pei G, Lv W, Liu S, Lv X, Qiu G, Preparation of synthetic rutile from reduced ilmenite through the aeration leaching process, Chemical Engineering and Processing - Process Intensification (2019), doi: https://doi.org/10.1016/j.cep.2019.107774

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Preparation of synthetic rutile from reduced ilmenite through the aeration leaching process Junyi Xiang1,2,* 1

College of Materials Science and Engineering, Chongqing University, No. 174

Shazheng Street, Shapingba District, Chongqing 400044, China. 2

Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and Advanced

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Materials, Chongqing University, No. 174 Shazheng Street, Shapingba District, Chongqing 400044, China. *[email protected]

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Guishang Pei1

College of Materials Science and Engineering, Chongqing University, No. 174

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Shazheng Street, Shapingba District, Chongqing 400044, China. Wei Lv1

College of Materials Science and Engineering, Chongqing University, No. 174

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1

Songli Liu3 3

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Shazheng Street, Shapingba District, Chongqing 400044, China.

College of Materials Science and Engineering, Yangtze Normal University, No. 16

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Juxian Avenue, Fuling District, Chongqing 408100, China. Xuewei Lv1,**

Collgege of Materials Science and Engineering, Chongqing University, Chongqing

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1

400044, China. **[email protected]. Guibao Qiu1 1

College of Materials Science and Engineering, Chongqing University, No. 174

Shazheng Street, Shapingba District, Chongqing 400044, China.

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Graphical abstract

Highlights 

An improved Becher process for the production of synthetic rutile was

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proposed.

Catalysts significantly enhanced the aeration process.

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Magnetite can be steadily formed as by-product with the existence of AQ-2,6.

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The recovery rate of titanium was improved by magnetic separation method.

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

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Abstract

The traditional aeration leaching process for the upgrading of reduced ilmenite is

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extremely sluggish and needs significant improvements in its efficiency. An improved aeration leaching process which comprises reduction, aeration with a catalyst, wet

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magnetic separation, and further leaching, was used for the preparation of synthetic rutile in the present study. The effects of NH4Cl concentration, HCl addition, 9, 10anthraquinone-2, 6-disuifonic acid, disodium salt (catalyst AQ-2, 6) addition, temperature, stirring speed, liquid/solid mass ratio, and leaching time on the metallic iron removal rate from the reduced ilmenite were studied. The bench-scale experiment results reveals that the aeration leaching efficiency can be significantly improved by

adding 2.0% HCl or 0.2% AQ-2, 6 in NH4Cl solution, and the fine magnetite (Fe3O4) particles can be steadily produced with the addition of AQ-2, 6. The pilot-scale experiment results show that the synthetic rutile can be upgraded to ~86% of TiO2 with 0.2% AQ-2, 6 addition within 8 hours of aeration leaching, and the recovery of TiO2 can be improved by 3% when replace the traditional hydrocyclones with the wet magnetic separation. The improved process can not only shorten the leaching time but also possible to improve the quality of the products by accurate classification.

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Keywords: Becher process; synthetic rutile; ilmenite; magnetic separation; aeration.

1. Introduction

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Synthetic rutile (SR) is a chemically modified ilmenite, which has most of the non

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titanium components removed. SR is used as feed stock for the production of titanium dioxide pigment via chlorination-oxidation process, and titanium metal via

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magnesiothermic reduction process, or directly used as welding electrode covering [12]. SR can be manufactured by upgrading ilmenite feed to a titanium dioxide product

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using several commercial processes, namely: Benelite process, Ausptpac process, Murso Process, and Becher process [3-6]. Benelite process employs carbon thermo-

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reduction for conversion of ferric iron to ferrous state and then removed by hydrochloric acid leaching [3]. In the Austpac process, ilmenite ore is roasted to

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selectively magnetise the ilmenite, and removed the gangue minerals by magnetic separation, followed by a hydrochloric acid leaching process [4]. Oxidation and reduction of ilmentie followed by hydrochloric acid leaching is utilized by Murso process [5]. In the above mentioned process except Becher process, all involve a high acidity leaching step at elevate temperatue which is high energy consumption and serious corrosion.

The Becher process was investigated and developed by R.G. Becher in Western Australian Government Chemical Laboratories in 1961, and now is operated commercially. In this technology, the ferrous and ferric content of the ilmenite concentrate is first reduced to metallic iron in a rotary kiln at ~1500C using bituminous coal as both fuel and reductant (CO). These changes can be explained by considering the following reactions. (1)

Fe2TiO5 + TiO2 + CO → 2FeTiO3 + CO2

(2)

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Fe2Ti3O9 + CO → 2FeTiO3+TiO2+CO2

2FeTiO3 + CO → FeTi2O5 + Fe + CO2

(3)

FeTi2O5 + CO → Fe +2TiO2 + CO2 (2≤x≤3)

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nTiO2 + CO → TinO2n-1 + CO2 （n＞4）

(4) (5)

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The materials resulting from this step, termed reduced ilmenite (RI), consist mainly of grains of metallic iron embedded in a matrix of titanium dioxide [6]. Then the

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metallic iron is removed from the reduced ilmenite by an accelerated corrosion reaction using oxygen dissolved in a NH4Cl aqueous solution, formed a mixture of

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fine (0.1–10 μm) iron oxides (Fe2O3, Fe3O4) and oxyhydroxides (alpha or gamma FeOOH), and coarse titanium dioxide mineral particles . Finally, wet separation of the

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fine iron oxide from the coarse titanium mineral particles is done using hydrocyclones and spiral classifiers. The separated coarse titanium mineral particles is termed

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standard synthetic rutile (SR) [7-8]. In the aeration leaching step, the removal of metallic iron from the RI grains is essentially a redox reaction, and can be represented by the following anodic and cathodic reactions: Fe → 2Fe2+ + 4e (anodic reaction)

(6)

O2 + 4H+ + 4e → 2H2O (cathodic reaction)

(7)

Oxidation of ferrous ions: 2Fe2+ + 4OH– +1/2O2 → Fe2O3·H2O+H2O

(8)

Becher process has been found by several commercial companies, such as Iluka Resources Limited, due to its environmental advantages, as well as the reduced capital and operating costs [9]. It only consumes small amounts of reagents and the wastewater is almost neutral. Furthermore, the byproduct “red mud” can be utilized as raw material for the ironmaking process, or upgraded to iron oxide red. However, the

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rusting step in the Becher process is a batch process and can be extremely sluggish (some industrial batch aeration time up to 22 hours) and needs significant

improvements in its efficiency [9-10]. Some reports show that the rusting process can

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be accelerated by improving the air or oxygen partial pressure [11], adding ethylene

diammonium chloride [12], using multidendate ligands or carbonyl compounds [13],

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hydrochloric acid [14], and anthraquinone derivatives substituted with sulfonic acid

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groups [10, 15]. A class of redox catalysts based on anthraquinone derivatives, was identified as being effective catalysts for aeration by research conducted at CRISO

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minerals with industrial support such as by Iluka Resources. The work presents a study of the preparation of synthetic rutile from the reduced

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ilmenite through the aeration leaching process. The effects of hydrochloric acid and anthraquinone-based redox catalyst on the improving the efficiency of the aeration

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stage were evaluated. The effects of the aeration leaching parameters such as NH 4Cl concentration, leaching temperature, stirring speed, liquid/solid (L/S) mass ratio are also investigated.

2. Experimental 2.1. Materials

The reduced ilmenite samples used in the present study were provided by a local producer and used as received. The reduced ilmenite was produced by carbothermic reduction of ilmenite concentrate in a rotary kiln at about 1150 C for approximately 8 h. The ilmenite concentrate used in this plant is a natural weathered beach sand with the main phases of pseudobrookite and ilmenite. The chemical compositions of ilmenite concentrate and reduced ilmenite are reported in Table 1. All the other reagents used for leaching and chemical analysis were of analytical grade and of

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purity greater than 99.9%.

2.2. Experimental procedure

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The bench scale aeration leaching experiments were performed in a 1000 mL

beaker. The details of the experimental apparatus are illustrated in Fig. 1. A

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determined amount of solution was pre-heated to the operating temperature prior to

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adding the charge of reduced ilmenite. During aeration, a certain amount of distilled water was added to the beaker in order to compensate for solution losses by

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evaporation and entrainment of mist in the exit airflow. After a specific aeration time, the fine iron oxides were separated from the relatively coarse titanium minerals by wet screening [10] or wet magnetic separation methods depending on the type of iron

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oxide product.

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The pilot scale aeration leaching experiments were carried out in a 3000 L glass lined reactor fitted with a steam jacket, axial down-pumping stirrers, baffles, four gas inlet tubes, and thermocouples (as shown in Fig. 1). After aeration, the fine iron oxides were separated using hydrocyclones or wet magnetic separator depending on the type of iron oxide product. Then the coarse titanium mineral particles were washed with 0.5% sulfuric acid in a 2000 L glass lined reactor at about 70 C for 30

min. After leaching and filtration, the solid samples were dried and calcined at 900 C for 1 h.

2.3. Characterization The metallic iron content of the mineral solids was determined by potassium dichromate titration method in FeCl3 solution. Other elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (ICAP 6000,

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Thermo Fisher Scientific, USA). The aeration leaching extent was defined as follows:  (%) = (1-CSRWSR / CRIWRI ) 100%

(9)

Where CRI is the content of metallic iron in the reduced ilmenite, WRI is the weight of

WSR is the weight of the synthetic rutile product.

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the reduced ilmenite, CSR is the content of metallic iron in the synthetic rutile product,

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The particle size of the samples was analyzed with a laser diffraction particle size

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analyzer (Mastersizer 2000, Malvern Instruments Ltd., UK) using water as dispersant. The mineralogical composition of the samples were identified by X-ray powder diffraction (XRD) performed using a Rigaku D/max 2500 diffractometer (Japan) with

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Cu Kα radiation (λ=0.154 nm, 40 kV, 150 mA) at a scanning rate of 0.3 °/s. The

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preliminary quantitative phase analysis of the X-ray diffractograms was carried out using the search/match software program MDI Jade 6.0 applied to the ICDD Powder

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Diffraction File database. The SEM/EDS analysis of the samples was performed using a scanning electron microscope (VEGA 3 LMH; TESCAN) equipped with an energy dispersive X-ray spectroscope (Oxford).

3. Results 3.1. Characterization of the reduced ilmenite

After reduction in a rotary kiln, large portion of ferrous iron and ferric iron was reduced to metallic iron, giving an analysis of 29.46 wt.% MFe and 6.67 wt.% FeO in the reduced sample (as shown in Table 1). The XRD patterns of the reduced ilmenite are shown in Fig. 2. The major crystalline phases ilmenite (FeTiO3) and pseudobrookite (Fe2TiO5) are transformed into metallic iron (Fe), rutile (TiO2) and ferrous pseudobrookite (FeTi2O5). The preliminary quantitative analysis of the X-ray diffractograms reveals that the weight

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concentration of metallic iron, ferrous pseudobrookite, and rutile in reduced ilmenite are found to be 27.6 (±1.9)%, 30.3 (±2.2)%, and 42.1 (±2.9)%, respectively.

The back-scattered electron (BSE) and elemental distribution images of the cross

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sections through the reduced ilmenite particles are shown in Fig. 3. The BSE images reveal mainly three distinct areas that appear as white-gray, light gray, and dark gray,

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which can be identified as metallic iron, titanium oxide, and epoxy, respectively. Parts

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of small metallic iron particles are dispersed in the matrix of titanium oxide, while others metallic iron particles with a large scale are coated on the surface of the matrix

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due to the agglomeration and growth of the small metallic iron particles. The elemental distribution images reveal that the reduced ilmenite particles are enriched

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with Ti, O and Fe, and the distribution of Fe is consistent with the distribution of white-gray area in Fig. 3 (b).

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The particle size distribution of the reduced ilmenite is represented in Fig. 4. The reduced ilmenite has a large particle size in the range of 60-500 μm with a d(0.1) of 105.531 μm, d(0.5) of 183.865μm, d(0.9) of 323.496 μm, and volume mean diameter D[4, 3] of 201.397 μm.

3.2. Bench-scale aeration leaching

The metallic iron was removed by the aeration leaching process, and the effect of leaching conditions was investigated. The main parameters that influence the leaching of metallic iron are NH4Cl concentration, hydrochloric acid concentration, leaching temperature, L/S mass ratio, stirring speed, additives, and leaching time.

3.2.1. Effect of NH4Cl concentration The effect of NH4Cl concentration on the removal of metallic iron is studied by

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varying the NH4Cl concentration in the range of 0.5-2.0 using 100g of reduced ilmenite with a L/S mass ratio of 6:1, air flow of 0.6 L/min, stirring speed of 400 rpm, at 70 C for 10 h.

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Fig. 5(a) shows that the leaching extent of metallic iron from the reduced ilmenite

significantly increased from less than 20% to 43.78% with the increase of NH4Cl

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concentration from 0.5% to 1.0%, and then slightly increased to approximately 47.85%

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with further increased of NH4Cl concentration to 2.0%. Ammonium ion, acting as a buffer, prevents excessively high local pH, which might otherwise cause precipitation of iron (II) hydroxide before the iron (II) could diffuse from the rutile matrix; this is

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termed as “in-situ rusting”. Furthermore, the NH3 produced stabilises Fe2+ and allows it to diffuse out from pores into the bulk solution. The chloride ion also help to break

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down any passive film which might form during aeration [6, 11]. The positive effect

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of NH4Cl on the removal of metallic iron is significant in the NH4Cl concentration in the range of 0.5-1.5%, while weak exceed 1.5% NH4Cl concentration.

3.2.2 . Effect of hydrochloric acid concentration As mentioned above, after 10 hours of aeration leaching, the leaching extent of metallic iron is very low through a typical becher process. This could be due to the

passive film which formed before or during aeration, such as the oxidation of the metallic iron in the superficial layer, hinder the mass transfer between the rusting agent and metallic iron. It’s reported that the aeration leaching rate can be significantly improved by adding a little amount of hydrochloric acid [14]. So in this series of tests the effect of hydrochloric acid concentration on the rate of removal of metallic iron is examined in 1.5% NH4Cl solutions with four different hydrochloric acid concentrations (0, 1, 2, and 3%).

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As shown in Fig. 5(b), the removal of metallic iron significantly enhanced by the addition of hydrochloric acid in the NH4Cl solution. It’s obvious that the leaching extent of metallic iron rapidly increased from approximately 47% with no

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hydrochloric acid addition to 79.63% with 1% hydrochloric acid addition, 90.97% with 2.0% hydrochloric acid addition, and slightly increased to 93.2% with 3%

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hydrochloric acid addition.

3.2.3. Effect of leaching temperature

The effect of leaching temperature on the removal of metallic iron from the reduced

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ilmenite is investigated by varying the leaching temperature in the range of 50-80 C and keep other conditions same as above. As shown in Fig. 5(c), after 10 h of aeration

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leaching, the leaching extent of metallic iron slightly increased from 85% to

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approximately 91% with the increase of leaching temperature from 50 C to 70 C, whereas decreased to 87.2% with further increasing leaching temperature to 80 C. The diffusion of ions in the aqueous solution can be enhanced by the increase of leaching temperature, which further accelerating the rusting rate. However, the solubility of oxygen will slightly decrease with increasing temperature, which is not favorable for the reaction on the cathodic and the oxidation of ferrous ions.

3.2.4. Effect of stirring speed The effect of stirring speed on the removal of metallic iron from the reduced ilmenite is determined by varying the stirring speed in the range of 200-500 rpm in a solution consisting of 1.5% NH4Cl and 2.0% hydrochloric. As shown in Fig. 6(a), the leaching extent of metallic iron significantly increased from 83.42% to approximately 91% with the increase of stirring speed from 200 rpm to 400 rpm, and almost kept

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stable with further increased in stirring speed to 500 rpm. External diffusion would increase with the increase of stirring speed, which leads to the aeration leaching

efficiency improved. In order to minimize the contribution of natural convection to

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the mass transfer, 400 rpm was chosen as the optimum stirring speed. Furthermore,

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3.2.5. Effect of L/S mass ratio

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the slurry would splashed over the beaker with stirring speed exceeding 500 rpm.

The effect of L/S mass ratio on the removal of metallic iron is considered by using four different ratios (3:1, 6:1, 9:1, and 12:1).

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The L/S mass ratio has a noticeable effect on the aeration leaching rate. As shown in Fig. 6(b), there was a sharp increase in the leaching extent of metallic iron when the

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L/S mass ratio increased from 3:1 to 6:1, which results in the decrease of pulp

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viscosity and the improvement of external diffusion. However, there was only a small increase in the leaching extent of metallic ion over the L/S mass ratio of 6:1. Therefore, 6:1 was chosen as the optimum L/S mass ratio in the present study. Excessive higher L/S mass ratio not only decreases the overall equipment effectiveness but also increases the financial cost.

3.2.6. Effect of additives and aeration leaching time The effect of AQ-2, 6 on the removal of metallic iron is investigated by varying the addition of AQ-2. 6 from zero to 0.8% in 1.5% NH4Cl solution and keeping other conditions same as above. As shown in Fig. 6(c), the leaching extent of metallic iron significantly increased from less than 40% to 93.25% with increase in AQ-2, 6 concentration from zero to 0.1%, then slightly increased to approximately 97% up to 0.2% AQ-2, 6 concentration, after which almost keep steady.

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Fig. 7 presents the effect of additives and leaching time on the removal of metallic iron. There are two stages for the upgrading process: a sharp increase in the leaching extent of metallic iron at about 5 h, and then a slower increase in longer aeration time.

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When aeration in pure NH4Cl solution (1.5%), the metallic iron leaching extent

increased to 46.82 wt% in the first 10 h, while slowly increased to 61.34 wt% in the

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second 10 h. Adding 2.0% hydrochloric acid lead to 76.77 wt.% of metallic iron

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leaching extent in the first 5 h and slightly increases to approximately 91 wt.% with further extending aeration time to 10 h. The aeration process can be further enhanced by adding 0.2% AQ-2, 6 in 1.5% NH4Cl solution, the metallic iron leaching ratio

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rapidly increased to 91.82 wt.% just in the first 5 h. Adding both 2.0% HCl and 0.2% AQ-2, 6 in 1.5% NH4Cl solution leads to 94.3 wt.% metallic iron removal rate in the

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first 5 h and approximately 99% at 10 h.

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Also interesting to note is that the aeration slurry and dried iron oxides have different colours with different additives. Fig. 8 shows photos of the aeration slurry and dried iron oxide by-products. The slurry shows dark orange (Fig. 8a) and dried iron oxide shows reddish brown (Fig. 8b) when conducted in pure 1.5% NH4Cl. Furthermore, color of the slurry conducted with HCl (2.0%) in 1.5% NH4Cl is nearly the same. The reddish brown oxides are a mixture of iron oxide (Fe2O3, alpha or

gamma FeOOH), which also known as “red mud”. However, magnetite (Fe3O4) was the only oxide detected when conducted with any proportional AQ-2, 6 in 1.5% NH4Cl (as shown in Fig. 9), the slurry (Fig. 8c) and dried iron oxide (Fig. 8d) show black color.

3.3. Pilot scale aeration leaching Several pilot scale arearation leaching experiments were conducted based on the

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above results. The slurry used for this series of experiments consisted of 300 kg reduced ilmenite, 1500 L water, 1.5% NH4Cl, and 2.0% hydrochloric acid or 0.2% AQ-2, 6. An air flow of 40-60 m3/h and temperature of 60-70 C was maintained

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through the slurry. After a determined time of aeration leaching, the coarse titanium mineral particles were washed with 0.5% sulfuric acid at 70 C for 30 min then dried

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and calcined. As shown in Fig. 10, after leaching the dried synthetic rutile shows light

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gray color while roasted sample shows golden red with trace gray. Table 2 presents the chemical composition of the upgraded products. When

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aeration leaching in 1.5% NH4Cl and 2.0% hydrochloric acid solution for 15 h, the TiO2 content in the synthetic rutile has been upgraded from 58.20% to 84.22%, and

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the total iron content decreased from 34.65% to 5.86%. When aeration leaching in 1.5% NH4Cl and 0.2% AQ-2, 6 solution for 8 h, the TiO2 content in the synthetic rutile

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increased to 85.70%, and the total iron decreased to 4.82%. However, the recovery of TiO2 has been increased from 87.93% to 92.45% when AQ-2, 6 replaced hydrochloric acid. As mentioned above, when aereation leaching in NH4Cl solution with AQ-2, 6, the by-product was magnetite (Fe3O4) rather than other iron oxides. Therefore, the titanium mineral particle can be separated from the aeration leaching slurry by magnetic separation method. As shown in Table 2, the recovery of TiO2 can be

improved from 92.45% to 95.60 when replacing hydrocyclones with magnetic separation.

3.4. Characterization of product The XRD patterns and the major minerals presented in the calcined synthetic rutile are presented in Fig. 11. The major phase in the calcined synthetic rutile is rutile (TiO2) with minor pseudobrookite (Fe2TiO5) and silicon dioxide (SiO2). The preliminary

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quantitative analysis of the X-ray diffractograms reveals that the weight concentration of rutile, pseudobrookite, and silicon dioxide are found to be 81.2 (±5.3)%, 16.5 (±1.1)%, and 2.3 (±0.2)%, respectively.

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The SEM photographs of the calcined synthetic rutile are shown in Fig. 12. The surface of synthetic rutile particles is loose and porous. A large portion of synthetic

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rutile particles still maintain their shapes and particle size, but a small part of particles

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broke into pieces. Higher magnification image shows numerous porous reticular microstructure and surface topography. The core of synthetic rutile grains also shows porous due to the removal of metallic iron. The elemental distribution maps reveal

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that the synthetic rutile is enriched with Ti and O, while little with Fe, Si, Ca, and Mg.

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4. Discussion

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According to the original description of the aeration leaching process, the oxidation of the metallic iron in the reduced ilmenite is usually completed in 14-16 hours [8]. The leaching extent of metallic iron from the reduced ilmenite only reached to ~62% with 20 hours of aeration leaching in this study. This may be due to the formation of a passivation layer on the surface of the metallic iron in a long time of preservation, that reduced the corrosion rate of the metallic iron. It’s reported that the removal rate of

metallic iron can be increased when using ethylenediammonium chloride as catalyst, but the degree of the impact is not too much [12]. Furthermore, the reaction rate also can be improved at elevated temperatures (130 C) and pressure (300kPa O2). However, it leads to an increased in-situ rusting [11].

The removal rate of metallic iron can be significantly improved by adding a little amount of hydrochloric acid. The anodic reaction of metallic iron in NH4Cl solution

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can be enhanced as the decrease of pH value. The passivation layer on the surface of metallic iron can be destroyed as the existence of H+ and Cl-. Furthermore, the

cathodic reaction also can be improved by the increase of concentration of H+.

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However, more iron will dissolve rather than be precipitated with the addition of hydrochloric acid. The dissolved iron not only lowers the recovery of iron oxide but

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may also make the reutilization of the final solution more difficult [16]. The aeration

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process also can be significantly accelerated by adding a little amount of redox catalyst 9, 10-anthraquinone-2, 6-disuifonic acid, disodium salt (catalyst AQ-2, 6) in

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NH4Cl aqueous solution (as shown in Fig. 6). It’s reported that such catalysts facilitate electron transport from the metallic iron in the reduced ilmenite to the oxygen. Their redox potentials lie between those for the metallic Fe to Fe2+ and O2 to H2O couples

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such that the anthraquinones oxidise metallic iron and the semi-quinones or

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hydroquinones are oxidised by oxygen [8]. The major advantage of the AQ-2, 6 is that it is able to act as an additional and recyclable oxidizing agent. It’s reported that the dissolution rate of gold in cyanide solutions can be improved by 50% with the addition of 3.0 mM anthraquinone-2-sulphoric acid (AQ-2). Gold also can be oxidized with the addition of AQ-2 in dilute cyanide solution in oxygen-free atmospheres [17]. AQ also able to act as a further additional oxidizing agent in the

“in-situ” production of hydrogen peroxide [18]. The oxidation of metallic iron in the reduced ilmenite can be significantly shortened from nearly 20 hours to about 5 hours with the addition of AQ-2, 6 into the NH4Cl aqueous solution (as shown in Fig. 7). However, the effect of combined hydrochloric acid and AQ-2, 6 does little for further improving of the rusting rate. Thus, AQ-2, 6 alone is more preferable as the catalyst for the aeration process. Another important is the formation and utilization of the by-product. It has been

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reported that it is not always possible to control the type of oxide formed during aeration, several kinds of oxides can be formed in a slightly varying conditions, such

as hematite (α-Fe2O3), lepidocrocite (γ-FeOOH), maghemite (γ-Fe2O3), goethite (α-

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FeOOH), magnetite (Fe3O4) and several other phases [10, 19]. The black oxide magnetite (Fe3O4) is industry’s preferred oxide because of its superior settling and

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handling properties. The formation of magnetite (Fe3O4) is highlighted because it can

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be easily separated from the titanium mineral particles by utilizing magnetic separation. The magnetic separation results are also shown that the recovery of TiO2 almost increased by 3% compared with hydrocyclone separation method. Furthermore,

Table 2).

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the grade of the coarse titanium mineral particles is also slightly higher (as shown in

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Based on the above results and analysis, an improved process for the production of

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synthetic rutile through Becher method is proposed. As shown in Fig. 13, ilmenite ore is first reduced to reduced ilmenite in a rotary kiln, followed by aerating in NH 4Cl solution with addition of AQ-2, 6 catalyst, after 10 or fewer hours of aeration, the slurry was then classified by single or double wet magnetic separators into three kinds of compounds, namely, weak and non-magnetic, medium, and strong magnetic particles. The weak and non-magnetic particles are titanium minerals with trace

metallic iron. The medium magnetic particles are insufficiently aerated reduced ilmenite, which should be returned to the prior aeration step. The strong magnetic particles are fine magnetite (Fe3O4), which can be used as iron-making material or iron oxide pigment. The weak and non-magnetic part is filtered and dried, and is referred to as standard synthetic rutile. It also can be further upgraded though acid leaching step, which can dissolve the unreacted metallic iron, remaining ferrous oxide, and other acid soluble metal. Wet magnetic separation was suggested to separate the

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magnetite (Fe3O4) and titanium mineral particles, because the traditional separation methods, such as hydrocyclones and spiral classifiers are only suitable for minerals

with a large particle size. However, a small portion of ilmenite particles acquired from

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different mine lots and beneficiation processes are very fine, which would fall into the overflow particle size. Furthermore, the particle size of the samples would slightly

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decrease due to the friction between the particles and the crush of the stirring paddle.

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While wet magnetic separation not only can minimize the loss of titanium minerals but also separate the aerated insufficient particles and return to the aeration process, which helps to improve the quality of the products. The increased cost of the catalyst

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can be offset by the advantages of the proposed process, such as short production time,

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high production capacity and device utilization, and low power consumption.

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5. Conclusions

Under traditional aeration leaching conditions, no more than 65% of metallic iron

removed from the reduced ilmenite with 20 hours of leaching. Aeration efficiency can be significantly improved by adding 2.0% hydrochloric acid and/or 0.2% AQ-2, 6 to 1.5% NH4Cl aqueous solutions, approximately 77% and 91% of metallic iron leaching extent can be achieved in 5 hours of leaching, respectively. When

hydrochloric acid was used, the optimum aeration leaching conditions used in this study was 1.5% NH4Cl concentration, 2.0% hydrochloric acid addition, 70 C aeration leaching temperature, 400 rpm stirring speed, 6:1 L/S mass ratio, and not less than 10 hours of retention time. The retention time can be double shortened with the same conditions when AQ-2, 6 was used. Furthermore, magnetite (Fe3O4) can be steadily formed as the by-product by adding a little amount of AQ-2, 6. The mineralogy result indicates that the surface of the synthetic rutile is loose and porous.

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A small proportion of the products may fall into overflow particle size that cannot be

effectively recovered by traditional separation methods. The pilot scale aeration

leaching experiments reveals that the reduced ilmenite can be upgraded to ~86% of

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TiO2 by adding 0.2% AQ-2, 6 to 1.5% NH4Cl solution with 8 hours of leaching.

Furthermore, the recovery of TiO2 could be increased from 92.5% to 95.6% by

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replacing the traditional hydrocyclones with wet magnetic separation method.

ACKNOWLEDGMENTS

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The authors are indebted to the anonymous reviewers for their constructive suggestions for this manuscript, as well as the Project funded by China Postdoctoral

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Science Foundation (grant numbers 2018M640898) and the Natural Science

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Foundation of China (grant numbers 51674055) for sponsoring part of this work.

REFERENCES [1] Mackey, T.S., 1994. Upgrading ilmenite into a high-grade synthetic rutile. JOM, 46(4), 59-64.

[2] Samal, S., 2011. The dissolution of iron in the hydrochloric acid leach of titania slag obtained from plasma melt separation of metalized ilmenite. Chemical Engineering Research and Design, 89(10): 2190-2193. [3] Chen, J., 1974. Beneficiation of titaniferous ores. US Patent 3825419. [4] Walpole, E.A., Winter, J.D., 2002. The Austpac ERMS and EARS processes for the manufacture of high-grade synthetic rutile by the hydrochloric acid leaching of ilmenite. Proceedings of Chloride Metallurgy 2002, 2, Montreal, Canada

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(2002), 401–415. [5] Sinha, H.N., 1972. Ilmenite upgrading by the Murso process. Light Metals, AIME-TMS, New York, USA, 261-274.

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[6] Farrow, J.B., Ritchie, I.M., Mangano, P., 1987. The reaction between reduced

ilmenite and oxygen in ammonium chloride solutions. Hydrometallurgy, 18(1),

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21-38.

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[7] Becher, R.G., 1963. Improved process for the beneficiation of ores containing contaminating iron. Australian Patent 247110. [8] Bracanin, B.F., Cassidy, P.W., Mckay, J., Hockin, H.W., 1972. The development

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of a direct reduction and leach process for ilmenite upgrading. In: Rostsell, W.C. (Ed.), Light Metals 1972. TMS, New York,USA, 209–259.

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[9] Geetha, K.S., Surender, G.D., 2000. Experimental and modelling studies on the

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aeration leaching process for metallic iron removal in the manufacture of synthetic rutile. Hydrometallurgy, 56(1), 41-62.

[10] Bruckard, W.J., Calle, C., Fletcher, S., Horne, M.D., Sparrow, G.J., Urban, A.J., 2004. The application of anthraquinone redox catalysts for accelerating the aeration step in the becher process. Hydrometallurgy, 73(1–2), 111-121.

[11] Jayasekera, S., Marinovich, Y., Avraamides, J., Bailey, S.I., 1995. Pressure leaching of reduced ilmenite: electrochemical aspects. Hydrometallurgy, 39(1), 183-199. [12] Ward, J., Bailey, S., Avraamides, J., 1999. The use of ethylenediammonium chloride as an aeration catalyst in the removal of metallic iron from reduced ilmenite. Hydrometallurgy, 53(3), 215-232. [13] Kumari, E.J., Bhat, K.H., Sasibhushanan, S., Mohan Das, P.N., 2001. Catalytic

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removal of iron from reduced ilmenite. Minerals Engineering, 14(3), 365-368. [14] Guo, Y.F., Liu, S.S., Xiao-Wen, M.A., Tao, J., Qiu, G.Z., 2010. Rusting kinetics of metallic iron in reduced ilmenite strengthened by hydrochloride. Chinese

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Journal of Nonferrous Metals, 20(10), 2038-2044.

Australian Patent 7170063.

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[15] Fletcher, S., Horne, M.D., 2000. Treatment of titanium-containing material.

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[16] Guo, Y.F., Xia, L., Qiu, G.Z., Tao, J., 2012. Strengthening of metallic iron rust in reduced ilmenite. Zhongnan Daxue Xuebao, 43(3), 797-802. [17] Trindade, R.B.E., Monhemius, A.J., 1993. The use of anthraquinone as a catalyst

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in the cyanide leaching of gold. Minerals Engineering, 6(6): 565-574. [18] Kamachi, T., Ogata, T., Mori, E., Iura, K., Okuda, N., Nagata, M., Yoshizawa, K.,

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2015.Computational Exploration of the Mechanism of the Hydrogenation Step of

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the Anthraquinone Process for Hydrogen Peroxide Production. The Journal of Physical Chemistry C, 119(16): 8748-8754.

[19] Ward, C.B., 1990. The Production of Synthetic Rutile and By-product Iron Oxide Pigments from Ilmenite Processing. Ph.D. Thesis, Murdoch University, Perth, W.Australia.

Fig. 1 Figures of the aeration leaching apparatus: (a) bench scale experiment; (b) pilot scale experiment.

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Fig. 2 XRD patterns of the reduced ilmenite.

Fig. 3 Back-scattered electron (BSE) and elemental distribution images of the cross

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sections through the reduced ilmenite particles: (a-c) BSE images; (d-i) elemental

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distribution maps of the areas in b.

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Fig. 4 The particle size distribution of the reduced ilmenite.

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Fig. 5 Effect of (a) NH4Cl concentration, (b) hydrochloric acid concentration, and (c)

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leaching temperature on the leaching extent of metallic iron from the reduced ilmenite.

Fig. 6 Effect of (a) stirring speed, (b) L/S mass ratio, (c) AQ-2, 6 dosage on the

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metallic iron removal from reduced ilmenite.

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Fig. 7 Effect of addictives and leaching time on the leaching extent of metallic iron.

Fig. 8 Photos of the aeration slurry and dried iron oxide (a–b) without AQ-2, 6, and

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(c–d) with AQ-2, 6.

Fig. 9 XRD patterns of the by-product (leaching with addition AQ-2, 6).

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Fig. 10 Photos of the synthetic rutile (a) dried sample (b) calcined sample.

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Fig. 11 XRD patterns of the calcined synthetic rutile.

Fig. 12 Secondary electron (SE) and elemental distribution images of the calcined synthetic rutile: (a-c) SE images of the synthetic rutile grains; (d-f) SE images of the cross sections through the synthetic rutile grains; (g-l) Elemental distribution maps of the areas in e.

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leaching process.

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Fig. 13 Improved flowchart for the production of synthetic rutile through aeration

Table 1. Chemical compositions of ilmenite ore and reduced ilmenite (wt.%) TFea

MFeb

Sample

TiO2

Ilmenite ore

50.83 31.36 19.29 0

Reduced ilmenite 58.2

FeO

34.65 6.67

SiO2

MgO

Al2O3 CaO

P2O5

S

1.69

1.54

1.28

0.31

0.07

0.02

29.46 1.94

1.61

1.84

0.36

0.06

0.02

a

Total iron (TFe); bMetallic iron (MFe).

(wt.%)

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Table 2. Chemical compositions and TiO2 recovery of the upgraded products

Separation method

TiO2 TFec SiO2 TiO2 recovery

0.15% NH4Cl, 2.0% HCl, 15 h

Hydrocyclones

84.22 5.25 2.79 87.93

0.15% NH4Cl, 0.2 % AQ-2, 6, 8 h

Hydrocyclones

85.70 4.82 2.84 92.45

0.15% NH4Cl, 0.2 % AQ-2, 6, 8 h

Magnetic separation 86.17 4.56 2.90 95.60

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Aeration leaching conditions

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Total iron (TFe).