In case of high alumina, due to high melting temperature of alumina the slag produced became very viscus. This slag sometimes became difficult to drain from blast furnace. Therefore furnace irregularities may be happened such as non dry furnace, lower blast intake, increased hot blast pressure, channeling, slipping and also tuyere burning etc. For that reason, To handle this problem, alumina concentration should decrease in slag by increasing slag volume. Slag volume increase means the more energy loss, more flux addition etc. For that reason alumina should be as low as possible in iron ore and the ration of alumina/ silica should be less than unity. 

High silica, low alumina iron ores have no specific disadvantage in blast furnace iron making, unless silica is so high that it results in large slag volume. Low Al2O3 in ore facilitates and helps in maintaining lower alumina in slag without increasing slag rate and keeps it fluid. However in sponge iron making (DRI) the SiO2 / Al2O3 ratio should preferably be 1. Higher SiO2 in ore causes accretion formation in DRI kiln. A balanced Al2O3 content enables easy dislodging of formed accretions.   

Tolerance to silica and alumina in iron ore depends on the process and ore size. Lump +6mm particles can be fed directly into the blast furnace while fines (generally -6mm) need to be agglomerated via sintering for example. The sintering process is sensitive the high alumina which causes increased melt phase viscosity resulting in reduced strength. In the blast furnace the slag composition can vary in alumina content depending on how the furnace is operated. I have seen some operations in China run very high alumina but the silica content is usually 3-4 times that of alumina. No operation will run with a single feed source so the availability of blending ores also plays a role in the selection process.

Hi- Silica Iron ore fines are very useful for sinter making, as they are used to dilute lower silica iron ore during blending and resulting to the desired sinter chemistry.

Hi: The large dielectric constant leads to larger capacitance values for a given thickness (compared to silicon nitride).