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Review of Manganese Processing for

Production of TRIP/TWIP Steels, Part 1: Current Practice and Processing

Fundamentals

Richart Elliott, Kenneth S. Coley, Sina Mostaghel, and Mansoor Barati

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Citation (published version)

Elliott, R., Coley, K., Mostaghel, S. et al. JOM (2018) 70: 680. https://doi.org/10.1007/s11837-018-2769-4

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1

A Review of Manganese Processing for the Production of TRIP/TWIP Steels

Part 1: Current Practice and Processing Fundamentals R. Elliott1*, K. Coley1,2, S. Mostaghel3, and M. Barati1

1Department of Materials Science & Engineering, University of Toronto, Toronto, Canada

2McMaster Steel Research Centre, Department of Materials Science and Engineering, McMaster

University, Hamilton, Canada 3Hatch Ltd., 2800 Speakman Drive, Mississauga, Canada

*Contact: richard.elliott@mail.utoronto.ca

Abstract

The increasing demand for high performance steel alloys has led to the development of TRansformation

Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) alloys over the past three decades.

These alloys offer exceptional combinations of high tensile strength and ductility. Thus, the mechanical

behaviour of these alloys has been a subject of significant work in recent years. However, the challenge of

economically providing Mn in the quantity and purity required by these alloys has received considerably

less attention. To enable commercial implementation of ultra high Mn alloys it is desirable to lower the

high material costs associated with their production. Therefore, the present work reviews Mn processing

routes in the context of the chemical requirements of these alloys. The aim of this review is to assess the

current state-of-the-art regarding the reduction of manganese ores and provide a comprehensive reference

for researchers working to mitigate the material processing costs associated with Mn production. The

review is presented in two parts: Part 1 introduces TRIP and TWIP alloys, current industrial practice and

pertinent thermodynamic fundamentals; Part 2 addresses the available literature regarding the reduction of

Mn ores and oxides, and seeks to identify opportunities for future process development.

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1. Introduction

Manganese is the third most common constituent of steel. Annual global production of metallic manganese

exceeded 13 million tonnes in 2013, with over 90% of mined manganese ores consumed by steelmaking.

The remainder is used in applications such as batteries, fertilizers, catalysts, pigments and non-ferrous

alloys [1–3]. Manganese is used in steelmaking as both a deoxidizer and an alloying agent [4]. Most steel

grades (i.e. carbon and construction steels) average <1% Mn whereas current high manganese steel grades

(i.e. Hadfield and 200-series stainless steels) contain 11-13% Mn. Despite accounting for <3% of the 1.6

billion tonnes of steel produced annually, these steels consume 20% of all annual manganese production

[1,5].

TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) steels also require an

Mn content (4-30%) that is significantly higher than that required by most commercial alloys. However,

the mechanical properties of these alloys makes them attractive for a range of applications, including the

light-weighting of automobiles [6]. Widespread utilization of these steel grades will significantly increase

the global demand for manganese alloying materials. The properties of these TRIP/TWIP steels and the

challenges involved in their fabrication and forming have been, and continue to be, discussed at length.

The cost of producing the required manganese units, however, has received considerably less attention.

The present work is Part 1 of a two-part review of Mn processing in the context of ultra high manganese

alloys. This portion of the review addresses the properties and chemistry of TRIP and TWIP alloys, the

composition of Mn ores, the current state of commercial Mn processing, and the fundamental

thermodynamics of Mn ore reduction. Part 2 will review the available literature regarding experimental

reduction studies performed on Mn ores and oxides, and assess opportunities for new process development.

The objective of this review is to assist in the design of a more energy efficient and economical processing

strategy for high Mn alloys.

1.1 TRIP and TWIP Steels

The behaviour of TRIP and TWIP steels is an active area of research and has been reviewed regularly and

thoroughly over the past decade [6–16]. These reviews discuss at length the relationship between material

properties and chemical composition of various high manganese alloys, typically with a focus on the

ultimate tensile strength and total uniform elongation. To enable a discussion of the material processing

requirements for these alloys, some relevant key concepts are outlined here.

Figure 1 summarizes the tensile strength and ductility characteristic of most commercial steel types,

including TRIP and TWIP. The Advanced High Strength Steels (AHSS) are divided into three generations.

The 1st generation (<25 GPa%) encompasses a range of low Mn (typically <3%) containing alloys,

characterized by their high tensile strength and comparatively low ductility. This includes a low-alloy

TRIP or TRIP-assisted classification. The 2nd generation (>50 GPa%) is composed of high Mn (>10%)

TRIP and TWIP alloys with exceptional ductility and high tensile strength. The 3rd generation (>35 GPa%)

represents a compromise between cost and performance, sacrificing elongation and tensile strength for

reduced material costs and manufacturability [15,17].

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Figure 1 General range of tensile strength (MPa) and total uniform elongation (%) possible

within a variety of steel alloys. Alloys listed include: Interstitial Free (IF), Interstitial

Free High Strength (IF-HS), Bake Hardening (BH), Carbon-Manganese (CMn), High

Strength Low Alloy (HSLA), Dual Phase (DP), Complex Phase (CP), TRIP,

Martensitic (MART), Quenching & Partitioning (Q&P), and TWIP. Adapted from

Bouaziz et al. (2011), Keeler and Kimchi (2014), Lee and Han (2015), and Hernandez

et al. (2016) [7,15,18,19].

The exceptional strength and ductility of TRIP and TWIP steels are the result of the deformation

mechanisms for which they are named. In TRIP alloys, retained austenite undergoes a transformation to

ε-martensite under strain [8]. In TWIP alloys, the application of strain induces the formation of mechanical

twins, strengthening the alloy through a ‘dynamical Hall-Petch effect’; as the average grain size is

decreased, the strength of the alloy correspondingly increases [12,20]. In both cases plasticity is maintained

even at high strain.

The deformation mechanism of an alloy correlates with its Mn content. TRIP behaviour is typically

reported in steels with Mn content of 1-10%, while TWIP is reported in steels with >10% Mn. However,

high Mn content alone is insufficient to produce TRIP or TWIP behaviour: an Fe-30Mn alloy displayed

neither phenomena, despite its exceptionally high manganese content [7].

The deformation mechanism of an alloy also correlates with its Stacking Fault Energy (SFE), a parameter

which characterises the energy required to create a defect in a material [21]. In high Mn steels, an SFE

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<20 mJ/m2 favours TRIP while a SFE of 20-50 mJ/m2 favours TWIP. An SFE >50 mJ/m2 suppresses the

formation of mechanical twins and the material will instead deform by slip [12,21,22].

To achieve the desired SFE, and thereby the desired mechanical performance, alloying elements may be

added to the steel. Additions of Al, Si and N have all been reported to increase the SFE, while C additions

result in a decreased SFE [8]. The effect of each wt-% alloy addition on the SFE per wt-% is typically

reported in the range of ±1-3mJ/m2, although higher values on the order of ±5-10mJ/m2 have been reported

[12]. The composition of TRIP/TWIP steels may also include additional elements such as Ni, Cr, Mo, and

V which act to stabilize the retained austenite [23–25].

Typical compositional ranges for both TRIP and TWIP alloys are given in Table 1. Both alloys require

tight control over their SFE, and therefore over their composition, in order to achieve the desired

mechanical performance. While the maximum Mn concentration for a TRIP alloy is listed as 2.5% in Table

1, many alloys with higher Mn concentrations (i.e. 3rd generation AHSS) may still exhibit TRIP behaviour.

The dependence of TRIP and TWIP behaviour on low concentrations of alloying elements, including tramp

elements, represents one of the major challenges limiting their commercialization. In particular, it is costly

to achieve the required Mn content in the alloy while maintaining sufficiently low C levels.

Table 1 Typical range of major alloying elements for TRIP and TWIP steels [8,13,22,26–42].

Alloy Mn (%) Al (%) Si (%) C (%) Fe (%)

TRIP 0.2-2.5 0-2 0.04-2.5 0.09-0.4 bal.

TWIP 15-25 2-4 2-4 0.15-0.6 bal.

The complex chemical composition of TRIP/TWIP alloys also complicates the fabrication and forming of

these steels. In particular, it has been noted that the high strength (even at elevated temperatures), tendency

to form non-metallic inclusions and banded structures, scale formation, and wide brittle-temperature range

of these alloys present challenges in cold and hot-rolling due to the formation of cracks and surface defects

[43–46]. Issues with the weldability of these alloys have also been reported [47,48]. Overcoming these

challenges has spurred development in a number of near-net-shape casting technologies, such as Strip

Casting and Belt Casting Technology [49,50].

In 2011, POSCO became the first steelmaker to produce a TWIP steel on the commercial scale [51].

However, to the authors’ knowledge, this remains the only commercially available TWIP alloy. A variety

of 3rd generation TRIP, Dual Phase (DP) and Q&P alloys are readily available from a range of steelmakers

[52–54].

1.2 Mineralogy of Manganese Ores

To meet the chemical requirements of TRIP or TWIP alloys, it is necessary to consider the composition

of Mn ores. Most commercially relevant manganese ores occur as oxides, formed by the weathering of

Mn-rich carbonates or silicates [3]. The 30 most common Mn minerals have been summarized in Table 2.

The eight most important (as evaluated by Post, 1999) and commercially relevant (as evaluated by Christie,

2011 and Wellbeloved et al., 2012) are marked (*) [3,55,56].

Many manganese minerals are analogues of other common minerals (e.g. hematite [Fe2O3] and bixbyite

[Mn2O3]), and their Mn atoms are often partially substituted by these other elements (i.e. bixbyite may be

[(Fe,Mn)2O3])[3]. Commercial ores may also contain Mn as manganese silicates (e.g. braunite

[3(Fe,Mn)2O3•MnSiO3]) or manganese carbonates (e.g. rhodochrosite [MnxCO3]). Positive identification

of a given mineral phase is complicated by significant overlap in diffraction patterns between manganese

5

oxide minerals, necessitating the use of multiple analytical methods in concert [3]. The literature pertaining

to the processing of Mn ores primarily reports the presence of pyrolusite, braunite II, bixbyite, hausmanite,

and manganite. Excluding braunite II, these minerals form the series: MnO2, Mn2O3, Mn3O4, MnO.

Table 2 Important manganese minerals, their chemical formula and nominal manganese

content by mass [3,55–57]. While Mn is found in more than 300 minerals, most are

not commercially relevant; the principle commercial minerals are marked (*).

Mineral Chemical Formula Nominal Mn Content

(wt%)

Bementite Mn3Si2O5(OH)4 43.2

Birnessite (Na, Ca)Mn7O14•2.8H2O 50.9

Bixbyite* Mn2O3 55.6

Braunite 3(Fe,Mn)2O3•MnSiO3 48.9-56.1

Braunite II 3(Fe,Mn)2O3•CaSiO3 52.6

Chalcophanite ZnMn3O7•3H2O 48.2

Coronadite PbMn8O16 48.7

Cryptomelane* KMn8O16 55.8-56.8

Feitknechtite MnO•OH 62.5

Galaxite (Mn,Fe,Mg)(Al,Fe)2O4 31.8

Groutite MnO•OH 62.5

Hausmannite* Mn3O4 64.8

Hollandite BaMn8O16 42.5

Jacobsite Fe2MnO4 23.8

Lithioporite LiAl2MnO6(OH)6 17.5

Manganite* MnO•OH 62.5

Manganocalcite (Mn,Ca)CO3 <20-25

Manganosite MnO 70.9

Manjiroite NaMn8O16 61.7

Nsutite Mn(O, OH)2 45.4

Oligonite (Fe,Mn)CO3 23-32

Pyrochroite Mn(OH)2 61.8

Pyrolusite* MnO2 63.2

Ramsdellite MnO2 63.2

Rhodochrosite* MnCO3 47.6

Rhodonite MnSiO3 42

Romanechite Ba0.66Mn5O10•1.34H2O 50

Tephroite Mn2SiO4 54.4

Todorokite (Ca, Na, K)2Mn6O12•3.5H2O 49.4-52.2

Vernadite MnO2•1.4H2O 44-52

1.3 Production of Manganese Ores

Manganese ore is mined and processed globally, with ore mining countries typically exporting their

product to feed international steelmaking demand. These ores are categorized based on their manganese

content as: high-grade (>44% Mn), mid-grade (<44% and ≥30% Mn), and low-grade (<30% Mn) [2]. A

breakdown of global manganese ore production in 2013 is given in Figure 2. More than 60% of all

manganese ore is mined in the top three producing countries (South Africa, Australia, China); a similar

amount of ore is consumed by China alone, reflecting their dominance in the global steel industry [1,5].

6

As TRIP and TWIP steels have exacting purity requirements, the impurities contained within these ores

are also of interest. As P is detrimental to the performance of high Mn steels, the high P content (200-2000

ppm) of many Mn ores is of particular concern [58].

Figure 2 Global annual manganese ore production by country for 2013. Production is

normalized based on contained manganese units. All countries producing less than

500 000 mt per year grouped as ‘other’. All data from The International Manganese

Institute [2].

Terrestrial Mn ore reserves are estimated at 690 million tonnes and total Mn resources at 5.2 billion tonnes

[59]. Another significant resource exists in the form of marine manganese nodules: small (8-20 cm)

accretions of metal oxides found on seafloors worldwide. The Clarion Clipperton Zone (CCZ), a region of

the Pacific Ocean between Hawaii and Mexico contains an estimated 6 billion tonnes of manganese in this

form: equivalent to the combined terrestrial reserves [60,61]. Whereas current mining operations report an

average grade of 33.5% Mn, the CCZ nodules average about 28% Mn, equivalent to a low-grade terrestrial

ore[2,62] [61]. While this resource has been known for decades, legal and environmental concerns have

7

until recently prevented its exploitation [63,64]. New technologies have the potential to mitigate

environmental damage and the establishment of the International Seabed Authority in 1997 provides a

legal framework to manage the resource. However, the cost of producing Mn from this aquatic source is

estimated to be similar to that of terrestrial ores [61]. Therefore, it is unlikely that exploitation of marine

manganese nodules will significantly lower the cost of Mn.

1.4 Properties of Manganese Alloying Materials

Manganese is consumed by the steelmaking industry primarily in the form of alloys. These alloys are

classified based on their bulk composition and mode of production as: ferromanganese (FeMn),

silicomanganese (SiMn), and electrolytic manganese (EMM). They may be further classified based on

carbon content, i.e. high carbon (HC), medium carbon (MC) and low carbon (LC). The typical composition

of these alloying materials is given in Table 3 along with a relative price index. This price index shows the

total cost of each material relative to HC FeMn (Column 8) and the same comparison normalized based

on contained Mn (Column 9). For context, in 2015 HC FeMn prices were on the order of $0.93 USD/kg

while automotive scrap steel prices were $0.24 USD/kg [65].

Table 3 Typical chemical composition and price index relative to HC FeMn for manganese-

containing alloying agents and a high-grade manganese ore [58]. Price index is based

on AMM monthly averages from 2015 [65].

*may vary from 0.02 to 0.22 % depending on ore deposit

** may vary from 0.0005 to 0.09 % depending on ore deposit

The unit cost of Mn increases with increasing purity. As shown in Figure 3, this is due primarily to

increased processing costs and decreased process yields; processing costs account for less than 10% of the

HC FeMn price, but more than 50% of the LC FeMn price. The compositional requirements of TRIP and

TWIP alloys generally require that these more expensive LC materials be used. Although in theory total

Mn unit cost can be reduced by targeting either the cost of raw materials or the cost of processing, in

practice, greater opportunity lies in modifying the processing strategy; declining ore grades and availability

combined with increasing demand are unlikely to result in decreased ore costs.

Material

Typical Chemical Composition (wt%) Price Index

Mn Fe Si C P Other Rel. to

HC FeMn

Rel. to HC FeMn

& Mn-content

HC FeMn 78 bal. 0.3 6.8 0.20 - 1.0 1.0

MC FeMn 81 bal. 0.3 1.2 0.15 0.12 N 2.4 2.3

LC FeMn 82 bal. 0.6 0.2 0.15 0.12 N 2.7 2.6

MC SiMn 68 bal. 18 1.6 0.15 0.02 B, 0.2 Ti 1.2 1.4

LC SiMn 60 bal. 30 0.05 0.05 0.02 B, 0.2 Ti 1.8 2.3

EMM 99.8 0.001 0.002 0.002 0.001 ≤0.3 Se, 0.0005 H 4.2 3.3

Mn-ore 49 4 3 - 0.11* 0.008 B** 0.6 0.9

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Figure 3 Relative cost of commercially available Mn alloying materials, normalized based on

Mn content. Unit costs based on price index from Table 3. Distribution of ore and

processing costs estimated based on a common ore and assumed process yields of 95%

(HC FeMn), 75% (MC and LC FeMn), 85% (MC and LC SiMn), and 90% (EMM)

[66,67]. ‘Processing costs’ includes all components of the final market price, minus the

ore cost.

1.5 Production of Manganese Alloying Materials

The production of FeMn and SiMn by country is shown in Figure 4, normalized based on contained Mn

using the compositions given in Table 3 (see online supplementary material Figure S-1 for global

consumption of FeMn and SiMn). While alloy production occurs worldwide, more than 50% of all Mn

alloys are produced in China. Assuming 90% utilization of Mn ore for alloy production, these data indicate

a global average of 85% Mn recovery during processing.

In 2013, EMM accounted for an additional 1.1 million mt of production, 97% of which was in China

[68,69]. As EMM production is dominated by a single nation it has been omitted from Figure 4. While the

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purity of EMM makes it attractive for alloying, it is typically cost prohibitive to do so [58]. Current use of

EMM in steelmaking is limited primarily to the replacement of Ni in 200 series Stainless Steels [70].

While the mining of Mn ores and subsequent alloy production is dominated by a small number of countries,

the consumption of these alloys is widely dispersed. As the consumption of Mn alloys is closely linked to

the production of steel, many of the world’s top Mn producers are also major steel producers (notable

exceptions being South Africa and Norway, who export most of their product) [2].

Figure 4 Global annual production of manganese alloys for steel making for 2013. Production

is normalized based on contained Mn units using the average Mn grades for each

category listed in Table 3. All countries producing less than 100 000 mt per year

grouped as ‘other’. All data from The International Manganese Institute [2].

2. Commercial Manganese Processing Methods

Greater than 95% of all Mn used in steel is introduced in secondary steelmaking as either FeMn, SiMn or

EMM [57]. The production of FeMn and SiMn follow a similar pyrometallurgical route, while EMM is

10

produced via a hybrid pyro-hydrometallurgical process. In general, these processes are energy intensive

and produce significant waste, including significant volumes of slag and greenhouse gases. On the basis

of the average unit energy consumption noted below, and the production statistics provided in Section 1,

global energy consumption for the production of Mn alloys was on the order of 40 TWh. Some alternative

processes are under investigation but are not yet widely implemented.

2.1 Ferromanganese

Ferromanganese is produced primarily via the carbothermic reduction of oxidic manganese ores in a

Submerged Arc Furnace (SAF). This SAF route has largely replaced the historic use of Blast Furnaces for

ferromanganese production due to lower coking requirements, higher energy efficiency and longer

refractory life [71–73].

The SAF processing route may be operated using either the slag-discard method or duplex method. In both

cases Mn ore, a carbon source (i.e. coke), and fluxes are combined and heated via graphite electrodes to

1400-1450°C. This results in a C saturated FeMn alloy and a MnO-rich slag phase. In the discard method,

the furnace is operated to maximize Mn recovery to the alloy and the slag is discarded as waste; Mn

recovery is about 80%. The duplex method addresses this low yield by recovering the MnO-rich slag and

further processing it to produce SiMn; net Mn recovery is 85-95% [57]. The integrated duplex method is

depicted in Figure 5, along with options for producing MC FeMn and LC SiMn.

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Figure 5 Process diagram for a duplex FeMn-SiMn process. Typical stream compositions and

operating conditions are given where available [66,74]. Reproduced with permission

from [66].

The operation of the SAF is generally described by two zones: a low temperature zone above the burden,

and a high temperature zone in the heart of the furnace. In the low temperature region volatiles such as

water are removed, some higher order Mn oxides are thermally decomposed, and the remaining Mn oxides

are reduced by CO to MnO. In the high temperature zone, a two-phase mixture of solid MnO and liquid

slag is reduced at the surface of the coke bed, yielding the final alloy [73–76].

The efficiency of a FeMn process is characterized by its coking rate and energy consumption. These factors

are both heavily dependent on the extent of pre-reduction that occurs in the upper furnace zone. Inadequate

reduction of the Mn oxides by CO in the upper zone results in excess consumption of coke via the

endothermic Boudouard reaction (Eq. 1), increasing the coking rate and energy consumption while

lowering process efficiency [73].

C + CO2 = 2CO (1)

A typical FeMn smelting operation consumes on the order of 2100-3900 kWh of electricity per tonne of

alloy produced [66,75]. The coking rate of a FeMn smelting operation is in the range of 250-400 kg/t FeMn

[75]. Energy demand and coking rate vary depending on the ore grade and composition, with carbonate

ores having significantly higher energy requirements, on the order of 50% higher than a non-carbonate ore

with similar Mn content [77]. The operating efficiency for any ore can be improved by maximizing the

extent of prereduction [57,74,75,77].

Medium C FeMn is produced by blowing oxygen into HC FeMn in a manner analogous to the basic oxygen

furnace (BOF) in steelmaking. This manganese oxygen refining (MOR) is carried out at temperatures

>1750°C and can achieve C content as low as 0.5-1%, but it entails significant (up to 5%) Mn losses

through oxidation and vaporization [66]. Low C FeMn may be produced via the silicothermic reduction

of a Mn ore or slag containing >55% MnO. A high silicon (>22%) SiMn alloy is prepared and melted

together with the slag, and used as the reductant for the highly exothermic process [66].

2.2 Silicomanganese

Silicomanganese is produced either from FeMn slag as in the duplex process, or directly from Mn ores

employing a similar SAF process. In both cases, raw materials are charged to the SAF and processed at

1600-1650°C to produce a SiMn alloy. Specific power consumption is on the order of 3500-4500 kWh/t

SiMn; Mn yield is on the order of 75-85%. [66]

The Si/Mn ratio in the alloy and the concentration of MnO in the slag are a function of process temperature

and slag composition: higher operating temperatures and SiO2 content in the slag will both increase the Si

reporting to the alloy. Additionally, SiO2 saturated slag has the highest equilibrium MnO concentration

(~10%). Decreasing the SiO2 content or increasing the Al2O3 content in the slag (e.g., by increasing the

CaO content) can reduce the equilibrium MnO concentration, and therefore reduced Mn losses [78].

Low C SiMn is produced by the addition of FeSi to MC SiMn. As the Si content of the melt increases the

solubility of C decreases, forming SiC precipitates which segregate from the alloy based on their lower

density. The net result is a decrease in the C content of the alloy and an increase in the Fe and Si content

[66].

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2.3 Electrolytic Manganese Metal

The production of EMM has been reviewed in detail by Zhang and Cheng [67,79,80]. In this hybrid

pyro/hydrometallurgical process, Mn ores are roasted to produce MnO and subsequently leached to yield

a Mn-rich solution. Sulphuric acid is typically employed as the leachant, although leaching solutions

containing nitric acid, hydrochloric acid, and hydrogen peroxide have also been investigated [67].

Metallic Mn is then recovered by electrowinning. The leach liquor is treated to precipitate impurities (i.e.

Fe, Ni, Cu, Co, Zn) as either hydroxides or sulphates and then processed in an electrochemical cell, with

high purity Mn metal depositing on the cathode [81]. A similar method may be employed to produce

electrolytic manganese dioxide (EMD), which accounts for most Mn ores not consumed by the

steelmaking industry.

3. Alternative Methods

Given the energy intensive nature of metallic manganese production, and the high cost of producing low

C Mn units, there is significant incentive to explore other processing strategies. Current areas of research

include upgrading of low-grade ores, molten salt electrolysis (MSE) and solid-state reduction.

3.1 Thermal Upgrading of Low-grade Mn Ores

A two-step thermal upgrading strategy has been proposed for treating low-grade Mn ores. The ore would

be roasted to selectively reduce the contained oxides, forming non-magnetic MnO and metallic Fe;

magnetic separation would then allow the Fe to be stripped from the ore. Laboratory tests have

demonstrated the feasibility of this method. In one case, a 25 minute reduction roast at 600°C in a 30%

CO atmosphere allowed 50% of the Fe content of the ore (~6% of the initial ore mass) to be removed with

Mn losses of 5%; this Fe removal increased the Mn grade from 36% to 44%. For Mn-lean Fe-rich ores,

this may be a useful technique for reducing downstream processing costs [82,83]. However, the economic

case for this approach has not yet been proven.

3.2 Molten Electrolysis

Molten Salt Electrolysis has been proposed as an alternate refining step for HC FeMn to improve its

suitability for use in TWIP steels or similar alloys, providing a lower cost alternative to EMM [84–86].

While preliminary work has validated the technical feasibility of this pathway, the economic viability has

not yet been demonstrated. It is also theoretically possible to use molten oxide electrolysis (MOE) to

directly reduced a high-grade Mn ore [87]. To the authors’ knowledge this has not yet been demonstrated

in practice.

3.3 Solid State Reduction

A solid-state Direct Reduction (DR) scheme, followed by smelting in an Electric Arc Furnace (EAF), has

also been proposed for Mn ores [18,88–92]. Such a DR process would employ a reducing gas atmosphere

at temperatures below the melting points of both the ore constituents and reduction products. A similar

strategy is currently employed in the production of Direct Reduced Iron which allows iron ore to be

subsequently processed in an EAF, resulting in a net reduction in energy requirements relative to the BF-

BOF route [93].

The principle obstacle in such a process is the stability of MnO. It has been thoroughly demonstrated that

CO and H2 alone are inadequate to reduce MnO in the solid state [82,94–101]. It has, however, been

reported that a Mn carbide may be formed via reduction by a CH4-containing atmosphere at temperatures

in the range of 1000-1200°C [88,89,102–111]. Use of CH4 as a reductant could allow for lower temperature

13

operation, and would replace coke with natural gas. However, a CH4-based process would also yield a C-

rich product, which does not address the high cost of LC Mn.

4. Thermodynamic Considerations

The high cost of Mn alloying materials which meet the chemical requirements of TRIP/TWIP is directly

related to the thermodynamics of Mn oxide reduction. Despite the range of naturally occurring manganese

minerals, most thermodynamic and reduction studies of Mn ores consider only the simplified series of

manganese oxides, namely: MnO2, Mn2O3, Mn3O4, and MnO. Given that the manganese hydroxides and

carbonates will thermally decompose to simple oxides below 400°C, this is a reasonable simplification

[112–114]. The notable exception to this is the Mn silicate minerals, which decompose in the range of

900-1000°C, presenting some overlap with reduction temperatures for most studies [114]. The higher order

Mn oxides will also readily thermally decompose in an inert atmosphere or in air: MnO2 decomposes to

Mn2O3 in the range of 400-700°C; Mn2O3 subsequently decomposes to Mn3O4 at 900-1100°C. No thermal

decomposition of Mn3O4 to MnO has been reported [112,114,115].

4.1 Carbothermic Reduction

Conventional Mn processing is based on carbothermic reduction. Stoichiometric reaction equations for

simple Mn oxides with C are given in Equations 1 to 4. Analogous reactions for reduction by CO or H2

may also be written for Equations 1 to 3. Equation 5 gives the net reaction for the reduction of MnO2 to

Mn metal. Assuming no prereduction by CO, 2 mol of C are required per mol Mn; complete prereduction

can reduce this C requirement to 1 mol C per mol Mn.

2MnO2 + C = Mn2O3 + CO (2)

3Mn2O3 + C = 2Mn3O4 + CO (3)

Mn3O4 + C = 3MnO + CO (4)

MnO + C = Mn + CO (5)

MnO2 + 2C = Mn + 2CO (6)

The relative reducibility of Mn oxides, as well as FeO and SiO2, and their respective melting points are

summarized in the Ellingham Diagram in Figure 6. No melting point is given for MnO2 or Mn2O3 as they

will thermally decompose under ambient pressure prior to melting. The higher Mn oxides may be reduced

by C, CO, or H2; MnO may only be reduced by C. Assuming a carbon activity (ac) of 1, spontaneous

reduction of MnO by C requires a minimum temperature of 1340°C. To ensure economical reaction

kinetics, even higher temperatures are typically employed. Thus, carbothermic reduction is an inherently

high temperature energy intensive process.

14

Figure 6 Ellingham Diagram for manganese and its oxides, as well as FeO and SiO2. Data from

HSC Chemistry 6 and FactSage 7 [116,117]. For diagram with labelled PO₂, CO/CO2,

and H2/H2O axes, refer to online supplementary material, Figure S-2.

4.2 Solid-State Reduction

Reduction of MnO to Mn metal in the solid state (T<1244°C) requires a PO₂ of 10-19 atm, which is not

achievable in practice using CO or H2. Based on this Ellingham diagram, carbothermic or metallothermic

reduction (e.g. reduction by Si) in the liquid state are the only thermodynamically possible options for

reducing MnO. However, an ac greater than 1 could enable a solid-state reduction scheme. As shown in

Figure 7, elevated ac enables the formation of M7C3 or other Mn carbide phases at a practical PO₂ and solid-

state temperatures. The equilibrium line shown in Figure 7 shows the minimum PO₂ for a given carbon

15

activity (ac), assuming the sum of PCO, PCO₂, and PO₂ is 1. For ac=1 (i.e. equilibrium with a solid graphite

phase), it is thermodynamically unfavourable for a Mn metal or carbide to be formed at 1200°C.

Figure 7 Stability diagram for the Mn-O-C system at 1200°C constructed using FactSage 7

[118]. The equilibrium PO₂ for a given ac is shown based on the Boudouard reaction.

Ostrovski and Zhang have proposed that CH4 may be employed to establish an ac greater than 1 [119].

They suggest that on contact with the solid MnO phase, CH4 decomposes to form an adsorbed carbon

phase (Cad) and H2. As the Cad phase is in equilibrium with the CH4 atmosphere, its activity is significantly

higher than that of graphite, facilitating more rapid reduction. They define the activity of this Cad as

𝑎𝑐 =

𝑃𝐶𝐻4𝑃𝐻22⁄

(𝑃𝐶𝐻4

𝑃𝐻22⁄ )𝑔

(6)

where ac is the activity of carbon, and the subscript g denotes the ratio of CH4 to H2 in equilibrium with

graphite (ac=1). Based on this definition, even low concentrations of CH4 can generation exceptionally

high ac; at 1200°C, a 15%CH4-85%H2 atmosphere would generate an ac 50 times greater than that of

graphite, sufficient to reduce MnO in the solid state (refer to online supplementary material, Figure S-3).

However, this will necessarily result in the formation of a Mn carbide.

16

There is limited information available in the literature regarding the standard free energy for the formation

of the various Mn carbides, and what information is available is frequently in disagreement [120].

Although a variety of Mn carbides may be formed (i.e. Mn23C6, Mn3C, Mn5C2, Mn7C3), a recent assessment

of the Mn-C system has calculated that for C-rich conditions (mol fraction C > 0.3) Mn7C3 is the stable

phase at all temperatures [121].

5. Summary and Conclusions

The production of FeMn and SiMn for use in steelmaking applications is an energy intensive and costly

endeavour. The economical production of TRIP and TWIP alloys requires LC Mn units at a cost

comparable to that of Fe units. Currently no process exists which satisfies these requirements.

The processing and ore costs of LC Mn are of a similar magnitude; a decrease in either could provide the

necessary cost reduction to enable economical use of ultra high Mn alloys. However, a significant

reduction in Mn ore costs is doubtful. Upgrading strategies may be applied to low-grade ores to mitigate

future cost increases as high-grade ores are depleted, but are unlikely to reduce current ore prices.

Therefore, alternative low-cost processing strategies must be developed.

The high stability of MnO requires high temperatures when reduction is carried out using conventional

reductants. However, the use of CH4 to establish greater than unity ac could enable a solid-state process.

Such a solid-state reduction could be less energy intensive, greener (replacing coke with natural gas), and

lower cost. Additional refining would still be required to mediate the C content, and it is unclear how other

impurities affect solid-state reduction.

Supplying Mn materials suitable for use in ultra high Mn alloys is a significant challenge. The strict

compositional requirements of these alloys complicates the task of lowering the energy intensity of FeMn

and SiMn production. Part 1 of this review has briefly summarized current industrial practice and

introduced the fundamental obstacles to the development of lower cost processing options. Part 2 will

review available experimental works which address the reduction of Mn ores and oxides, and outline

potential avenues for process improvement and innovation.

6. Acknowledgements

The authors gratefully acknowledge the financial support of the Natural Science and Engineering Research

Council of Canada (NSERC, STPGP463252-14). Additional thanks to ArcelorMittal Dofasco, Stelco,

Praxair, and Hatch Ltd. for their in-kind support and technical expertise.

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