Target heart rate for fat loss

I am a 50 year old male weighing 200 Lbs Height 5'11"

I am exercising regularly on the treadmill for 1 hour 5-6 days a week

Time is not of importance to me and i can exercise for even more time if required.

My target is fat losss as i have belly fat. 

I wanted to know at what heart rate i should exercise for maximum fat loss ?

If i exercise at 120 BPM, i am barey starining myself and i can continue exercising endlessly and also without sweating.

However, when i push my hear rate to 140 BPM, i am sweating slightly and still breathing slightly hard only. I can easily finish 1 hour at this BPM with energy to spare.

So if if my aim is fat loss, should i continue to execrise at below 120 BPM or below 140 BPM for best results ?

Also i have read in other posts that exercising more then 60 minutes a day at a stretch does not give any additional benifit and 45-60 mins is the ideal exercising duration

Also should i do HIIT in a week instead of normal exercise to speed up metabolism ?

Thanks