At what temperature would a hotspot in a cement kiln become visible to the eye?

That depends to some extent on the human eye, which is variable. I can personally see them above 450 degrees C.

I would like to ask two questions (1) what influence has the fineness in the slurry in wet process in low/normal operation what happens in the kiln and chemistry change and the life of the refractary? (2) what happens in the kiln when burned the slurry with a moisture about 40-43 per cent especially the life of the refractory?

When the fineness of the slurry is low then potentially it will be more difficult to combine into clinker. This is not a chemistry change but the fact that there are larger particles means that conversion into the clinker minerals is more difficult as this is a solid state reaction and is dependent on the surface area and contact between the particles. Because the clinker combination is more difficult more fuel will need to be burnt and this could adversely affect the refractory life. The same applies to a high slurry moisture content. Because there is more water to evaporate in the kiln more fuel must be burnt. Again this can cause the refractory life to be reduced.