We settled on Viking helical gear pumps for molten bitumen for operating temp 140-190degC - presume these will do for molten sulphur also. As fas as I know, Viking are the only people who make these pumps with integral electrical casing heating elements - very useful for startup ops. External heat tracing of the pump casing can be unreliable and could lead to hot spots. Watch out for slippery interface issues between pump supplier and seal supplier - we used double seals with plan 53B flushing. If you want VFD speed turndown below 40% of rated speed, you'll need an electric motor with an independently driven aux cooling fan to cool the main motor. Good luck.