anions at the reaction interface in the desulphurisation 2

0 anions. This reasoning is based on the effect of adding FeO 73

to the CaO-SiOg system . The presence of FeO will increase the degree of polymerisation because the value of for FeO-SiO^ is greater than that for CaO-SiOg. This polymerisation will

2-

produce free 0 anions. The trend may be, therefore, that as FeO concentrations increase from slag KC1 (30.88 mass % 9 Table

p_

58) to KC5 (65.30 mass

%f

The implication of dispersed phase decarburisation is that ■the interfacial [ 0 ] and [ c ] respond in a manner that detract from

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good desulphurisation while the 0 anion concentration produced in the slag appears to be favourable for desulphurisation. This latter factor must be capable of suppressing the other effects in order for sulphur transfer from metal to slag to continue at the droplet-slag interface.

The hypothetical desulphurisation mechanisms, based on two dominant decarburisation mechanisms, highlight the importance of

2-

0 anions at the reaction interface in the desulphurisation 2-

process. If 0 anions are present during continuous phase controlled decarburisation transfer of sulphur from metal to slag is to be expected. The equilibrium interfacial sulphur concentration of the droplet will be low and will create a concentration gradient relative to the bulk sulphur

concentration of the droplet. This will ensure that sulphur in the droplet will be transported to the interface for reaction.

When the dispersed phase control decarburisation mechanism

2-

becomes dominant the 0 anion concentration of the slag must be sufficient to override the accompanying interfacial conditions that do not favour desulphurisation. If this is achieved the resulting interfacial equilibrium sulphur concentration of the droplet will maintain a concentration gradient relative to the bulk composition. This gradient will allow the continued transport of sulphur in the droplet to the interface for eventual transfer to the slag phase. On the basis that the interfacial conditions establish concentration gradients that allow metal to slag sulphur transfer throu^iout droplet-slag interaction, then the effect of mass transfer processes can be considered. It may, therefore, be expected that if the initial sulphur concentration in the droplet is sufficiently high, then the desulphurisation rate may be determined by conditions of

2-

continuous phase control, i.e. 0 anion transport control to the reaction interface, especially if the decarburisation process is

2-

controlling the availability of 0 anions at the reaction inter­ face. This is similar to the situation discussed earlier

(section 5.2.2) and would be expected to produce a constant desulphurisation rate. At lower sulphur concentrations in the droplet, dispersed phase control, i.e. S transport control in ihe droplet, would be expected to govern the rate of removal and a decreasing desulphurisation will be observed with reaction time.

2-

This situation is further encouraged if 0 anions at the surface increase as the decarburisation mechanism changes to one of

5.3.2 An Appraisal of the Observed Desulphurisation Behaviour

The ensuing discussion refers to the desulphurisation results given in Tables 8a to 8e and 13a to 17 and expressed diagrammatically in Figures 37 to I+3 and l+6a to 50d. The

appraisal considers the observed desulphurisation behaviour with respect to factors considered in the scenario developed in the previous section. Explanations are given to account for any differences between the hypothetical desulphurisation

mechanisms and observed desulphurisation behaviour.

(a) Reactions Involving Slag KC1 (38.06 Mass % CaO - 30.88 Mass % FeO - 30.80 Mass % Si CL - 0.27 Mass % S)

Droplets of Initial Sulphur Concentrations 0.512 Mass % m 0.UU0 Mass 0 . 302 Mass 0.120 Mass %

The desulphurisation results for these higher initial

sulphur concentration droplets are presented in Figures 37 to 1+0, Tables 8a and 8b.

Results for the "0.512 mass % S" droplet showed a linear change in sulphur content with time for about 2 minutes (Figure 37). Thereafter, the sulphur removal rate decreased with time. An identical scale was used to plot Figures 38 and 39 for the

"O.l+l+O mass % S'* and "0.302:mass % S" droplets. In both cases there was no initial linear change of sulphur concentration with time. The reaction rate decreased with time to a very slow

desulphurisation rate at 20 minutes. The "0.120 mass % S" droplet desulphurisation curve (Figure 1+0) was plotted using a different

scale and an exaggerated initial desulphurisation rate was evident relative to the higher sulphur concentration droplet results. It was clear, however, that in this experiment also, desulphurisation rate decreased with reaction time.

If the results discussed above had exhibited a well defined linear change of sulphur concentration with time, i.e. constant desulphurisation rate, then a period of slag phase controlled desulphurisation could have been interpreted. With the exception of the "0.512 mass % S" droplet for a very short period at the hi^h initial sulphur concentration, it is

difficult to envisage that such a mechanism was operative.

The general trend for the above results, i.e. decreasing desulphurisation rate with time, suggests at a first inspection that dispersed phased control conditions existed. In such a case the desulphurisation rate falls as a response to decreasing sulphur content of the droplet.

For the four sets of results, the best desulphurisation occurred within the 10 minute period of continuous phase

controlled decarburisation observed for this slag (Figures [+1+ and l+5)« During this decarburisation process the slag

composition at the droplet-slag interface assuming zero FeO concentration, would be CaO-O.L^xSiOg-O.OIxS. This composition is in the acid region but for the observed sulphur removal to

2-

have occurred, 0 anions must have been present at the inter­ face. Clearly, this situation is anomalous with respect to the hypothetical mechanisms proposed for the continuous phase

controlled decarburisation period. The actual desulphurisation achieved was too good to be accounted for by silicate

polymerisation in the CaO-SiO^ system. This suggests that the interfacial slag was not completely denuded in FeO and that during the first few minutes of reaction when desulphurisation rates were quite high, the interfacial composition must have been close to the bulk composition.

Retardation of sulphur removal also took place within the continuous phase controlled decarburisation period. This could be an indication of sulphur transport control in the droplet with the desulphurisation rate falling due to the sulphur content of the droplet decreasing.

After 10 minutes reaction the decarburisation process be­