During Bessemerisation of copper matte, silica is added so that the iron(II) oxide present is converted into

The matte contains $\mathrm{FeS}$, which is oxidised to $\mathrm{FeO}$. The acidic flux $\mathrm{SiO}{}_2$ combines with the basic $\mathrm{FeO}$ ($\mathrm{FeO}+\mathrm{SiO}{}_2\to \mathrm{FeSiO}{}_3$) to give a slag that floats and is removed. Iron stays as Fe(II) here, so the iron(III) silicate is wrong.