El desarrollo de la inflorescencia en Arabidopsis

Barcza (1979) determined experimentally that the Mn distribution ratio of the slag and

alloy,

(

)

Mn MnO

, was minimised for ferromanganese slags with a

MgO CaO

ratio not less than

3 and with a basicity ratio _      ₊ 2 SiO MgO CaO of about 1.33. The

(

)

ratio was used as

a measure of the degree of Mn recovered to the alloy. The lower this distribution ratio, the greater is the proportion of Mn in the alloy. This method proposed by Barcza is one way of classifying furnace performance. It seems that Mn recovery and the distribution of Mn between slag and alloy are measures by which performance can be evaluated.

In Urquhart (1980), physical properties were reviewed for high-carbon ferromanganese slags in terms of the slag basicity. Typical slags with basicities between 1.5 and 2.1 were

33 investigated at temperatures of around 1400 ºC. A basicity ratio is the ratio of basic

oxides to acidic oxides,

2 SiO

CaO MgO

MnO

B = + + . A basic oxide generally has a low cationic attraction for oxygen, whereas an acidic oxide has a stronger attraction. For this reason, metal oxides with large cationic radii such as those from the alkali and alkali-earth metals tend to de-ionise completely into cations and oxygen anions, donating free oxygens to oxygen accepting acidic oxides. It is common knowledge in slag chemistry that SiO2 is a polymerisation oxide. Oxide molecules such as SiO2, P2O5, B2O3, have a strong affinity for oxygen and are referred to as acidic oxides. When the concentrations of the acidic oxides increase, they ionise and form long (polymerized) chains that tend to make the slag more viscous and less conductive. Basic oxides such as MnO, CaO and MgO generally have the opposite effect and are called depolymerisation oxides. An excess amount of oxygen anions will break the oxygen bonds within a polymerised chain and form loose-standing, or non-bridging oxide molecules. Depolymerisation of a slag will therefore generally have the effect of decreasing the viscosity and increasing the electrical conductivity of a slag.

The change of slag properties (viscosity, electrical conductivity and liquidus temperature) with increasing component concentrations of typical ferromanganese slags are summarised in Table 1.

Table 1 The effect of increasing the concentration of each oxide in high-carbon ferromanganese slag on selected slag properties, for slags with typical molar basicities between 1.5 and 2.1.

Component Liquidus temp Viscosity Elec conductivity

MnO decrease * decrease increase

SiO2 Decrease increase decrease

CaO Increase slight increase slight increase

MgO Decrease decrease increase

Al2O3 Increase min at 15% Al2O3

#

max at 6-10% Al2O3

* this is not the case for very basic slags with high concentrations of MnO. The result that the liquidus temperature decreases with increasing MnO concentration is supported by the

experimental work of Warren et al. (1975).

#

34 Slag viscosities for typical high-carbon ferromanganese slags containing 5-20% MnO showed variations of viscosity of 1.58-0.69 Pa.s at 1400 ºC. Electrical conductivity varied between 6.6-22.7 S/m at 1400 ºC. Smaller variations in electrical conductivity and viscosity occur for more basic slags containing 10-35% MnO at 1400 ºC. In this instance, viscosity varied between 0.24-0.41 Pa.s and electrical conductivity varied between 27-50 S/m. “Due to the nature of slags a compromise is necessary between low viscosity and low electrical conductivity” (Urquhart, 1980). According to the viscosity model developed in Appendix E, it was found that viscosity measurement predictions for high-carbon ferromanganese slags with viscosity values greater than 0.1 Pa.s are highly variable and may be in error by up to 30%. Viscosity values predicted below 0.1 Pa.s may be even more variable. Note that 1 Pa.s = 10 poise.

For a manganese ore suited for electric furnace operation, the following property levels of the product slag would be favourable:

• Low slag to metal ratio to minimise the energy required to keep the slag in a molten state

• Intermediate viscosity and intermediate electrical conductivity to establish a good compromise between the lowest viscosity and conductivity levels possible

The conclusions arrived at in the experimental work of Channon and See (1977) explained the mineralogical effects of fluxing agents, CaO and MgO, on manganese recovery of medium carbon ferromanganese. It was found that manganese recovery decreased if the CaO:MgO ratio of the fluxing agents was decreased or if CaO was replaced by MgO in the flux used. It was also found that an increase in the slag

basicity ratio

2 SiO

MgO CaO+

(from 1 to 2.2) increased the amount of manganese to the

alloy and decreased the amount of silicon in the alloy. Both effects are desirable. These findings may not provide adequate support for the effects of CaO and MgO additions in high-carbon ferromanganese production, because lime or dolomitic additions are usually not required in the production of high-carbon ferromanganese. The medium carbon ferromanganese process is also different in that the

35 ferromanganese product is produced by silicothermic reduction and not by carbothermic reduction.

Equilibrium interactions between carbon-saturated Mn-Fe-Si melts and CaO-MnO-SiO2 slags

with and without Al2O3 were investigated in the work of Turkdogan and Hancock (1958).

Their conclusions are summarised in Table 2.

Table 2 Effect of compositional variations of CaO-MnO-SiO2 slags on alloy-slag distribution ratios

VARIABLES MANIPULATED CHANGE IN ALLOY-SLAG EQUILIBRIUM

Iron Basicity Alumina Si distribution ratio Mn distribution ratio

Fe CaO / SiO2 Al2O3 (SiO2) / [Si] (MnO2) / [Mn]

constant increase - Increases decreases

decrease constant - Increases decreases

- - decrease Increases increases

- increase constant, 20% increases slightly -

- increase constant, 0% Increases -

Fe-based - constant higher than MnO-based higher than MnO-based

36

CHAPTER 4

MODELLING METHODOLOGY