Production of Silico-Manganese in a Submerged Arc Furnace

Production of Silico-Manganese in a Submerged Arc Furnace

Silico-manganese (Si-Mn) is an alloy used for adding both silicon (Si) and manganese (Mn) to liquid steel during steelmaking at low carbon (C) content. A standard Si-Mn alloy contains 65 % to 70 % Mn, 15 % to 20 % Si and 1.5 % to 2 % C. Si-Mn alloy grades are medium carbon (MC) and low carbon (LC). The steelmaking industry is the only consumer of this alloy. Use of Si-Mn during steelmaking in place of a mix of high carbon ferro-manganese (Fe-Mn) alloy and ferro-silicon (Fe-Si) alloy is driven by economic considerations.

Both Mn and Si are crucial constituents in steelmaking. They are used as deoxidizers, desulphurizers and alloying elements. Si is the primary deoxidizer. Mn is a milder deoxidizer than Si but enhances the effectiveness due to the formation of stable manganese silicates and aluminates. It also serves as desulphurizer. Manganese is used as an alloying element in almost all types of steel. Of particular interest is its modifying effect on the iron-carbon (Fe-C) system by increasing the hardenability of the steel.

Si-Mn is produced by carbo-thermic reduction of oxidic raw materials in a three-phase, alternating current (AC), submerged arc furnace (SAF) which is also being used for the production of Fe-Mn. Operation of the process for the Si-Mn production is often more difficult than the Fe-Mn production process since higher process temperature is needed. The common sizes of the SAF used for the production of Si-Mn are normally in the range 9 MVA to 40 MVA producing 45 tons to 220 tons of Si-Mn per day.

In the carbo-thermic reduction of oxidic raw materials, heat is just as essential for reduction as C is, due to the endothermic reduction reactions and a deficiency of heat can cause incomplete. Good electrode penetration is therefore essential to have adequate heat to drive the reactions to completion.

The raw materials used in Si-Mn production mainly consist of Mn ore, high C Fe-Mn slag, quartzite, coke and coal, and fluxes (dolomite or calcite). The main source of Mn in raw materials for Si-Mn production is Mn-ore and Mn-rich slag from the high C Fe-Mn production. Different charge materials show different behaviour upon heating and reacting with C, which affects the consumption of coke and electrical energy, the quantity of slag and its composition, and the furnace productivity.

The amount of slag generation per ton of Si-Mn metal is mainly determined by the ore/slag ratio. Increasing share of Fe-Mn slag at expense of Mn-ore leads to larger slag/metal ratio in the Si-Mn production process. High volume of slag leads to an increased consumption of energy and probably to higher losses of metal inclusions in the final slag.

Mn ores generally contain unwanted elements which cannot be removed during the mining and processing stages. In this respect P (phosphorus) content is important because of the strict demand of this element in the Si-Mn alloy. Fe, P and As (arsenic) are reduced more easily than Mn and hence they go first into the metal. Their content in the final alloy is thus controlled by the selection of ores. The high C Fe-Mn slag is a very pure source of Mn since the easily reduced impurities in the ores have been taken up by the high C Fe-Mn metal during its production. The content of impurities, like P, in Si-Mn alloy is thus controlled, not only by the selection of Mn ore, but also by the relative amounts of Mn ore and high C Fe-Mn slag in the raw material mix.

A process temperature in the range of 1600 deg C to 1650 deg C is needed to obtain Si-Mn alloy with sufficiently high content of Si and for generation of the discard slag with low MnO. Fe-Mn slag has a relatively low melting temperature (around 1250 deg C) compared with Mn ore. Hence, a high share of Fe-Mn slag tends to give lower process temperature. When the Mn ore starts melting at around 1350 deg C, it normally contains a mixture of a solid and a liquid phase, where the solid phase is MnO. Further heating and reduction to 1550 deg C or more is needed before the melting ore mixes with the slag and flows freely. With a high share of Mn-ore in the mix, the surface temperature and process temperature in the coke bed zone is usually higher.

For Si-Mn production in SAF, C (coke and coal) is used as a reducing agent while the heat is supplied by the electricity. An electric current is sent through the charge, and heat is created according to P = R*I2, where P is the effect i.e. the heat created, R is the charge resistance and I is the current density.

In a SAF the electrode tips are submerged in a porous charge mix, and electrical energy is liberated by micro-arcing to a slag rich coke bed floating on top of a molten alloy bath. The heat requirement is supplied as electrical energy and the coke acts both as a reducing agent and electrical resistance element. The furnaces are circular, with an external diameter of 11.6 m and height of 6.2 m being typical of a 40 MVA furnace. The reactions which are taking place during the production of Si-Mn are given below.

The reduction of Si and Mn in the production of Si-Mn is takes place by the following series of different reduction steps.

(SiO2) + 2C = Si + 2CO (g)

(SiO2) + 2SiC = 3Si + 2CO (g)

(MnO) + C = Mn + CO (g)

(SiO2) + 2Mn = Si + 2(MnO)

(SiO2) + Si = 2SiO (g)

Mn = Mn (g)

The main equilibrium reactions which control the distribution of Si and Mn between the slag and Si-Mn alloy are the following.

(MnO) + C = Mn + CO (g)

(SiO2) + 2C = Si + 2CO (g)

Brackets indicate that the species are present in the slag, while underline indicates specie in the alloy. C is the carbon source, which can be dissolved in the alloy or solid, i.e. coke.

Complete slag / alloy / gas equilibrium requires simultaneous establishment of equilibrium for the two reactions. Both reactions are very dependent on the temperature and the CO pressure of the system. Higher temperatures give higher equilibrium content of Si in the Si-Mn alloy and lower MnO content in the generated slag. A low CO gas pressure also favour higher content of Si in the Si-Mn alloy and less MnO in the slag. Normally the CO gas pressure is quite close to 1 atmosphere in the submerged arc furnace. A combination of the above two reactions gives the partial slag / alloy equilibrium reaction, expressed by the following reaction.

2 (MnO) + Si = 2Mn + (SiO2)

This reaction is little dependent on temperature and independent of pressure and composition of the gas phase.

Excavation of a three phase 16 MW Si-Mn furnace provides information about the distribution of phases in the reaction zones in the furnace. The interior of the furnace is normally divided into two main areas namely (i) a pre-heating and pre-reduction zone where the charge components are still solid, and (ii) the coke bed zone where ore, slag and fluxes are molten. In this furnace, it was observed that Mn oxides are reduced almost totally from the Fe-Mn slag and the Mn ore at the top of the coke bed. This is shown in Fig 1 which is based on the excavation the above mentioned 16 MW furnace.

Fig 1 Zones of a silico-manganese furnace

Excavation of the furnace has indicated that an electrode tip position of around 600 mm above the metal bath has been suitable for good operation. MnO2 in the ore is decomposed early to Mn2O3, but further reduction to Mn3O4 by CO gas or by thermal decomposition is modest. Pre-reduction to MnO of any significance is only observed in the charge fines. The MnO-rich Fe-Mn slag is reduced almost to the final Si-Mn slag composition before substantial reduction of melting of Mn ore started on. Nearly all reduction of MnO is finished at the top of the coke bed. Dissolution and reduction of quartz have obviously taken place in the coke bed zone after the main reduction of Mn oxide is finished. Maybe the ’pick up’ of Si in the alloy is quite fast and it takes place as the Liquid metal trickles down through the coke bed towards the liquid alloy bath.

The reduction of the highest Mn oxide (MnO2) occurs in four steps. The first reduction step from MnO2 to Mn2O3 is reached at a temperature of greater than 450 deg C to 500 deg C and the second step Mn2O3 to Mn3O4 at greater than 900 deg C to 950 deg C and that both can be realized without reducing agent and only by the thermal decomposition. Both steps in the thermal decomposition show the range of the Mn oxide’s stability. The reduction ofMn3O4 to MnO is only possible by CO gas or solid C. The reduction of MnO by carbon at atmospheric pressure is only feasible at temperatures greater than 1410 deg C. To achieve full reduction the temperature needs to be even higher. The problem here is the high vapour pressure of Mn and its resulting strong evaporation. In the case of the Mn production process with its various reduction steps, MnO first dissolves in the slag phase from which Mn is reduced by solid C and moves into the metal phase. Here the non-ideal solutions of slag and alloy are of great importance. Up-to-date thermodynamic methods are to be used to determine phases and the energy balance based on the mass balance.

The distribution of Si between Si-Mn alloy and multicomponent MnO-SiO2-CaO-Al2O3-MgO slag is mainly determined by the process temperature, the SiO2 content of the slag and its R-ratio which is defined as (CaO + MgO) / Al2O3. For example, the equilibrium content of Si in the Si-Mn alloy increases by around 6 % if the R-ratio is reduced from 2 to 1 provided there is constant temperature and SiO2 content. The effect of temperature is also considerable. The equilibrium content of Si is increased by around 6 % per 50 deg C in the temperature range 1550 deg C to 1700 deg C. The equilibrium content of MnO in Si-Mn slag depends first of all on the temperature and secondly on the SiO2 content of the slag. At 1600 deg C the equilibrium content of MnO decreases from around 9 % at SiO2 saturation to a minimum of around 3 % to 4 % when the SiO2 content is reduced to around 40 % to 45 %.

The factors which influence the recovery of Mn beside temperature are (i) slag basicity [(CaO + MgO) / SiO2], (ii) CaO/MgO ratio, and (iii) Al2O3 content of the slag. Increasing temperature favours the endothermic reduction reaction both thermodynamically and kinetically. The effect of slag chemistry on the reduction of Mn oxide is more complex. The recovery of Mn is higher for the basic slag, due to higher MnO activity coefficients in the basic slag. The addition of lime to the raw material mix decreases the MnO saturation concentration and increases the MnO activity in the slag for the same MnO content. The result is a decreasing equilibrium MnO concentration in the slag and an increasing rate of reduction. An increase in the slag basicity above 1.1 has a less significant effect on the recovery of Mn. Much higher CaO and MgO contents result in higher slag viscosity which weakens the reduction of Mn. Al2O3 as well increases the slag viscosity and this can slow down the reduction reaction of Mn. To keep the slag well flowable, the concentration of Al2O3 in the slag is not to exceed 20 %.

During the smelting of Si-Mn, additions of dolomite or calcite to the charge increase the slag basicity, which improves the slag fluidity and facilitates the reduction of MnO from the slag. The typical SiO2 content in Si-Mn slag is in the range 35 % to 45 %. This slag has a liquidus temperature between 1300 des C and 1380 deg C, depending on the slag composition. Increasing the temperature or the SiO2 content in the slag and decreasing the (CaO + MgO) /Al2O3 ratio all increase the Si metal – slag partitioning coefficient.

For the improvement of slag fluidity, MgO content of the slag is required to exceed 7 %. A significant reduction of the MnO content of the slag can be achieved by increasing the proportion of MgO in the slag. This in turn improves the recovery of Mn.

Production of Si-Mn depends on the behaviour of Mn ore, Fe-Mn slag, quartz, and fluxes during smelting, reduction, and slag formation. These processes can be divided into the following three stages.

  • Heating and pre-reduction in the solid state – At this stage, which occurs at temperatures between 1100 deg C and 1200 deg C, Mn oxides are reduced to MnO, and iron oxides are reduced to metallic iron.
  • Formation of liquid slag and Mn oxide reduction – This stage is completed at the top of the coke bed where the temperature is uncertain and can be assessed as follows. The equilibrium Mn oxide content in the slag at 1500 deg C with Fe-Mn (before the start of the reduction of SiO2) in the range of 10 % to 25 % depends on the slag composition. Slag samples taken from the top of the coke bed during the furnace excavation contained around 10 % MnO. Such a low content of MnO in the slag is expected at temperatures greater than 1550 deg C. Further, liquid slag is required to have a low viscosity to percolate through the coke bed. Hence, the temperature at the top of the coke bed is expected to be in the range of 1550 deg C to 1600 deg C.
  • Reduction of SiO2 from the slag and further reduction of MnO – This stage occurs in the temperature range of 1550 deg C to 1650 deg C. The SiO2 content of slag in the coke bed is expected to be in the range of 40 % to 45 % which is the same as in the final slag. This indicates that the rate of quartz dissolution into the slag is close to the rate of SiO2 reduction from the slag and it maintains SiO2 concentration in the slag relatively constant. The MnO content in the slag is decreased further to around 5 % to 10 %. This slag is usually discarded.

The major component of the gas phase in the furnace interior is CO. Silicon monoxide (SiO) and Mn vapour only are present in small amounts. At low temperatures near the top of the furnace, the gas also contains CO2 and water vapour.

Reduction reactions change the ore composition, leading to changes in the melting temperature and other properties of the ore. The rate of these changes depends on the reduction rate, which is affected by several different parameters such as temperature, ore composition and morphology, properties of carbonaceous materials, and others.

The amount of slag per ton of Si-Mn is mainly determined by the ore/slag ratio. Increasing share of Fe-Mn slag at the expense of Mn ore leads to larger slag/metal ratio in the production process of Si-Mn. High volume of slag leads to an increased consumption of energy and probably to higher losses of Mn inclusions in the final slag.

The economics of Si-Mn smelting is enhanced by minimizing the loss of Mn as metal inclusions, as MnO dissolved in the slag, and by production of the alloy high in Si and low in C.

Low carbon Si-Mn with around 30 % Si is produced by upgrading standard alloy by addition of Si wastes from the Fe-Si alloy industry.

The specific power consumption for production of standard Si-Mn alloy from a mixture of Mn ore, high C Fe-Mn slag and Si rich metallic re-melts, can typically be 3500 kWh to 4500 kWh per ton of Si-Mn and is dependent first of all on the amount of metallics added to the feed. The power consumption increases with the Si content of the Si-Mn produced, and also with the amount of slag per ton of Si-Mn. Each additional 100 kg of slag produced consumes additionally around 50 kWh of electric energy. Around 100 kWh per ton of Si-Mn and some coke is saved if the ore fraction in the charge is reduced to MnO by CO gas ascending from the smelt reduction zone.