MATERIALS

4.4.1 Slag

The composition of FCS slag was calculated on the basis of a slag Fe/SiO2 ratio of 2.3

and CaO content between 20 to 30wt%. For the slag/brick experiments, approximately 10 wt% copper oxide was added to the slag as no copper was present during experiments for slag/metal equilibration. In the case of the distribution experiments, 4wt% Cu2O was added to

the slag, allowing further dissolution to occur during experiments. As discussed in Section 2.9, when FCS slag and copper metal are at equilibrium at an oxygen partial pressure of 10-6 atm. and 1300oC, the copper oxide content in slag is approximately 10wt%. In case of the distribution experiments, small amounts (1-1.5 wt%) of each minor element oxide (NiO, PbO, SbO1.5) were added to separate samples of FCS slags for equilibration with copper. The

amount of the element oxide added to slag was assumed to be within its saturation limits in FCS slag as discussed in the next section. Two master slags of varying CaO composition were used for the slag/brick experiments in order to determine whether slag composition had an effect on refractory wear. Since FCS slag is located near the dicalcium silicate surface, a CaO content close to the dicalcium silicate boundary was selected. The second slag composition had a CaO composition midway between the magnetite and Ca2SiO4 saturation boundaries.

The Fe/SiO2 ratio is both cases was kept constant at 2.3. For both slag/brick and minor

element distribution experiments, the master slags were prepared from CaCO3, which was

and AR-grade Fe2O3 and Cu2O. Appropriate amounts of each were ground together to mix

them, then packed in a magnesia crucible. The FCS slag was then melted for a total of 8 hours at 1300oC and oxygen partial pressure of 10-6 atm. in the tube furnace. The slag was initially held at 1000oC for 3 hours to allow the slag to equilibrate with the atmosphere whilst in a solid state and was then heated at 1300oC for 5 hours. The slag sample was lowered to the cool zone and cooled in a CO2/CO/N2 atmosphere for 10 minutes following the melting time,

and then was cooled in air to 25oC. The heating curve for preparation of the master slags in shown in Figure 4.4.1. 0 200 400 600 800 1000 1200 1400 0 2 4 6 8 10 12 14 16 Time (hrs) T e m p e ra tu re ( o C ) Slow Heating

Equilibration of slag and atmosphere whilst slag is in solid state

Melting Slag

Reaction Time Rapid Cooling

Slow Cooling

Figure 4.4.1: Heating curve for master slag preparation

The cooled slag was removed from the crucible, ground using a ring grinder and mixed to ensure homogeneity. The slag composition was analysed using the ICP-AES analysis technique and titrations were used to determine the ferrous iron (Fe2+) content. The composition of the slag is shown in Table 4.4.1 and Figure 4.4.2. The solubility of magnesia in FCS slag at 1300oC and an oxygen partial pressure of 10-6 atm. is not reported in literature; however analysis of the slag does indicate very minor dissolution of the magnesia crucible containing the slag melt.

Table 4.4.1: Slag composition used in both slag/brick and minor element distribution

experiments

Fe(T) Fe2+ CaO SiO2 MgO Cu2O Sb2O3 PbO NiO

FCS Slag MS-21 (incl. SbO1.5) 35.0 9.4 26.7 18.6 0.6 3.5 1.48 MS-22 (incl. PbO) 35.1 9.8 26.2 17.6 0.9 3.6 1.22 MS-23 (incl. NiO) 33.7 9.3 28.0 18.0 0.4 3.5 1.04 MS-24 (element free) 36.6 14.0 24.1 18.7 1.3 4.1 - - - MS-4 (slag/brick) 35.9 10.5 23.1 16.4 0.9 9.8 - - - MS-5 (slag/brick) 32.9 9.3 27.3 15.0 1.8 9.8 - - -

Calcium Ferrite Slag

MS-CF 46.7 - 17 - 1.2 15.1 - - - MS-₂₁ MS-22 MS-2₃ MS-24 _MS- 4 MS-₅

Figure 4.4.2: Liquid region of FeOx-SiO2-CaO slag with 10% Cu2O at 1300oC and an oxygen

partial pressure of 10-6 atm with experimental slag compositions (Kongoli, McBow and Yazawa, 2006).

Calcium ferrite slag was produced using calcined CaCO3, Fe2O3 and Cu2O heated in a

magnesia crucible in a CO2 atmosphere at 1300oC inside a muffle furnace for 4 hours.

Following the melting time, the slag was removed from the crucible and ground in a ring grinder to achieve intimate mixing. Magnesia crucibles are well known in literature to be attacked by calcium ferrite slag and thus to minimise attack of the crucible and the amount of magnesia dissolved in the slag, the slag was heated for 4 hours rather than the 8 hours used for FCS slag. The ferrite slag composition was also analysed using ICP-AES analysis. The composition of calcium ferrite slag is also shown in Table 4.4.1 (MS-CF).

4.4.2 Saturation of NiO, PbO and SbO

in FCS slag

If the amount of oxide added to the slag exceeds its saturation limits, it affects the reliability of the distribution data. The oxide which does dissolve takes part in distribution behaviour, but that which does not dissolve mostly remains in the slag. When the slag is analysed by wet chemical methods the reported analysis for the metal in slag will be comprised of both the dissolved and the undissolved oxide. This therefore systematically biases the distribution ratio in favour of the slag. Distribution data affected by saturation effects cannot be accepted as reliable. Grimsey et al. (1976), found nickel solubility of up to 10 wt% (13wt% NiO) in silica saturated iron silicate slag at 1300oC and an oxygen partial pressure of 10-7 atm. They found the solubility of nickel to increase with increasing oxygen partial pressure and thus it is expected that at the desired experimental conditions in the current research, that is, 1300oC and an oxygen partial pressure of 10-6 atm., nickel solubility in slag will be higher. Based on this and the expected thermodynamic prediction that the dissolution of nickel oxide in both the iron silicate and FCS slag is similar, the amount of NiO (>1.5 wt%) added to FCS master slag samples should be well within the saturation limits. Takeda et al. (1984) and Eerola et al., (1984) added approximately 1.5wt% nickel oxide to slag and did not report saturation issues. No data on the saturation limits of antimony and lead in FCS slag were found in the literature. Takeda et al. (1984), Eerola et al. (1984), Takeda et

al. _{(1983) and Acuna and Yazawa (1987) and Kim and Sohn (1991), who all studied the}

distribution of lead and antimony in calcium ferrite slag at 1250oC and an oxygen partial pressure of 10-6 atm., added approximately 1.5 wt% of lead oxide and antimony oxide, respectively, in slag and did not report any saturation issues. Therefore the amount of PbO or SbO1.5 added to FCS slag master samples was kept below 1.5 wt%.

4.4.3 Metal

Small amounts (1.2 – 1.4 wt%) of nickel, lead and antimony were alloyed with separate samples of copper for equilibration with FCS slag free of minor elements in the distribution experiments to reach equilibrium from both the metal and slag phases.

4.4.4 Refractory Brick

The slag was held in a crucible made from Radex DB-605 direct bonded magnesia- chrome refractory brick. The refractory was cut into a cylinder using a diamond tipped core drill of dimensions: 21mm OH x 12 mm OD. A 16mm IH x 5mm ID hole was then drilled into the sectioned brick using a 5mm silicon-carbide drill bit (Figure 4.4.2). Larger crucibles of dimensions 29mm OH x 18mm OD with 4mm wall thickness were made from the refractory brick in order to ensure that a slag/brick interface remained for inspections following experiments when both 32 hours contact time and a temperature of 1400oC were used. After cutting, the brick crucibles were ultrasonically cleaned for 1 hour to remove any small detached particles and dried for 2 hours at 60oC.

29 mm 25 mm 18 mm 10 mm 21 mm 16 mm 12 mm 5 mm

Figure 4.4.3: Dimensions of the magnesia-chrome refractory crucibles used in the slag/brick