64 Nordic Steel and Mining Review | Bergsmannen 2010/03

Production of MC FeMn and LC FeMn by de-carburization of HC FeMn is a metallurgical challenge due to the low oxygen potential and high vapor pressure of Mn. To reach carbon levels of below 2%, decarburization has to be performed at high temperatures. By diluting the partial pressure of CO - in the same manner as for stainless steel re-fining – decarburization is performed at more reasonable temperatures and lower final carbon contents can be achieved.

To minimize losses to slag and vaporiza-tion of Mn, the main focus during decarbu-rization is on temperature control. The CLU process is a unique tool in this respect. Th-rough the availability of steam as a process gas, dilution of the partial pressure of CO and the decomposition of steam favours the fundamentals for the decarburization.

Metallurgy fundamentals of manganese refining

The most important reactions to describe the decarburization process are:C + 1/2 O

2 (g) => CO (g) (1)

Mn + 1/2O2 (g) => MnO (2)

The sum reaction is expressed as:C + MnO => Mn + CO (g) (3)

Also important is the effect of the Mn eva-porationMn(l) =>Mn(g) (4)

The carbon removal is favoured by a high temperature, low CO partial pressure, high MnO activity in the slag, high C activity in the metal and low Mn activity in the metal. Added oxygen is mainly distributed between MnO and CO formation. The MnO formation

Ferromanganese Refining in Combined Blown CLU ConverterBy adapting models and technologies used in stainless steel making Uvån Hagfors Teknologi AB (UHT) has been able to create an environment for development, process control and production management which is unmatched in the ferroalloy sector. UHT’s solutions include the combined blown CLU­process, the UTCAS­process control system as well as general converter design details and production technologies common in stainless steelmaking but not yet adopted in the ferroalloy sector.

By Carl-Johan Rick , Kristina Beskow and Per-Åke Lundström; Uvån Hagfors Teknologi ABPhoto Uvån Hagfors Teknologi AB

Typical gas mix station for CLU.

65Nordic Steel and Mining Review | Bergsmannen 2010/03

Ferromanganese Refining in Combined Blown CLU Converter

increases gradually as the car-bon content decreases. The formation of MnO, along with the formation of CO, are ex-othermic reactions generating heat in the process.

Equilibrium partial pressu-res of CO and Mn for typical FeMn-refining systems have been calculated and published by Olsen et al [2]. The data have been used to create figure 1.

Figure 2 shows that decarbu-rization to a level of about 1.3 % C can only be reached if the process temperature is above 1,800 oC at a CO partial pres-sure of 1 bar. At 1,800 oC, the partial pressure of Mn is 0.44 bar.

By reducing the CO partial pressure to 0.5 bar the same carbon level, i.e. 1.3%, can be reached at a process temperature of 1,700 oC. At this temperature the partial pressure of Mn is only 0.28 bar.

The CLU Mn refining conceptThe CLU refining concept is a combined blowing technique in a converter vessel. Oxygen is blown through a top lance to the surface of the metal and oxygen,

At lower carbon concentrations, the car-bon removal requires a dilution of the CO partial pressure which is performed by blo-wing inert gas through the submerged tuye-res. The inert gas lowers the partial pressure of the carbon monoxide PCO , eq. 1. Due to the low vapor pressure of manganese, the Mn-gas also assists in lowering PCO.

PCO

=QCO

/(QCO

+QInertgas

+QMn

) x Ptot

eq. 1

The addition of an inert gas however also decreases the partial pressure of manga-nese, eq.2.

PMn

=QMn

/(QCO

+QInertgas

+QMn

)*Ptot

eq. 2

For this reason, the manganese vapor losses increase as the inert-gas amounts increase, given that the CO formation and temperature are constant.

As the carbon level decreases, the formed manganese vapor acts as an important in-ert gas and assists in lowering the partial pressure of CO. This enables top-blowing to lower carbon concentrations than would otherwise be possible - but at a cost of in-creased Mn losses.

The submerged oxygen reacts with car-bon according to mechanisms described in papers about the AOD-process [4]. Inert gas is blown in increasing I(g)/O2-ratio as the carbon content decreases. This gives the side effect that the partial pressure of Mn - PMn - decreases with an increasing eva-poration rate of manganese.

steam and argon is blown through sub-merged tuyeres from the bottom of the converter.

The oxygen blown through the top lance to the metal surface results in a hot spot [3] in the gas metal interface with a higher temperature than the bulk metal due to a high density of energy in that area, figure 2. For this reason the decarburization in the hot spot area can continue at a high rate to a lower level than would be possible at the bulk metal temperature.

Fig 1. Equilibrium pressures for finalization of FeMn refining.

Fig 2. The hot spot behaviour in top blown FeMn refining.

66 Nordic Steel and Mining Review | Bergsmannen 2010/03

By combining top and submerged blowing, the positive effect of the hot spot and high oxygen flow rates can be utilized at high car-bon contents with high carbon removal ef-ficiency and moderate manganese losses. At lower carbon contents, the decarburization is finalized with submerged blowing only.

Temperature controlAs discussed above, the formed CO along with the temperature forces Mn to vaporize. The vaporization itself is not only an in-situ

inert gas provider but an important tempe-rature controller in this system as it is stron-gly endothermic.

In the CLU process, steam is used as a process gas. When steam is injected in the metal it is reduced according to reaction 5 while consuming heat.

H2O(g) => H

2(g) +1/2 O

2 (g) (5)

Formed hydrogen gas acts as an inert gas while the oxygen takes part in the carbon oxidation.

Alloy Shape Typical Composition [1,2,3,4] Price in­dex [6]

World tonnes 2008 [5]

%C %Mn %Si %P %S %N

High carbon ferromanganese Lumps 7 78 0.3 0.2 0.01 ­ 1 4,795,000

Medium carbon ferromanganese Lumps 1.5 80 0.6 0.2 0.01 0.1 1.92 1,117,700

Low carbon ferromanganese Lumps 1.0 80 0.6 0.2 0.01 0.1

Ultra­low carbon ferromanganese Lumps 0.5 80 0.6 0.2 0.01 0.1

Nitrided ferromanganese Lumps 1 75 2 0.05 0.03 4

Electrolytic manganese Flakes or briquettes

0.05 98 1 0.01 0.04 0.05 2.71 1,180,000 *

(* figure estimated on different manga­nese.org sources)

Nitrided manganese Powder or briquettes

0.05 93 1 0.01 0.04 7

Silicon manganese Lumps 2 65 17 0.17 0.03 ­ 1.01 7,962,300

Low carbon silicon manganese Lumps 0.1 60 27 0.05 0.01 ­

A particular advantage of steam in this re-spect is that steam is relatively cheap compa-red to argon which is the main inert gas al-ternative in the lower carbon regimes of the process. As the solubility of nitrogen is high in manganese, nitrogen gas cannot be used unless nitrided products are being made.

Development opportunities During 2009 the UTCAS model package [8.9] was updated with a manganese vapori-zation model based on thermodynamic data

UTCAS process control system at Outokumpu Avesta Stainless.

Composition and availability of some manganese alloys.