There are times and places where trousers are just sensible. Horseback riding is one of those times, and while welding or grinding is another of those times. I too have ruined may shirts, and trousers with sparks from grinders and welding sparks. Even without all the whirring machinery and ladders, I'd not have worn a kilt to my old job due to sparks and mosquitoes. (These Delta skeeters can stab through a pair of new khaki twill pants, and they were so thick at times a dust mask was needed to keep from inhaling the little buggers.)

If you insist on wearing a kilt while welding and grinding, for one thing a leather kilt should be considered. It might get speckled, but is unlikely to burn.

Another possibility is to wear a pair of leggins or chaps that buckle on the leg higher than the bottom of the apron. If you can't find such leggins, contact a leathercrafter who specializes in Western style leather goods and talk to him about making you a set of Chinks, these are chaps that attach below the knee rather than to the belt.

I'd also suggest getting a longer apron, or having the one you have lengthened.