A Technical and Economic Comparison of Ball Mill Limestone Comminution with a Vertical Roller Mill

ABSTRACT

Wet flue gas desulfurization (WFGD), using limestone with forced oxidation (LSFO), is a common Flue gas desulfurization (FGD) process where limestone reacts with SO₂ to produce gypsum (CaSO₄.2H₂O). In South Africa, Eskom’s Kusile Power Station utilizes conventional wet ball milling to grind high-grade limestone (96% CaCO₃) to 95% passing 45 µm prior to use in the FGD scrubber. For future FGD installations under consideration, the high energy cost associated with ball milling, particularly when pursuing fine grinding, has become a crucial factor raising the need for a quest for alternative milling technology to reduce the costs. The vertical roller mill (VRM) which has gained wide acceptance in the cement industry and is used for grinding raw materials (mainly limestone), is a promising alternative. The VRM dries, grinds, and classifies ores and minerals within a single unit. A pilot-scale Loesche VRM was used to determine the power required to grind 100 t/h of the same limestone as used at Kusile (work index of 12 kWh/t) with a top size of 19 mm to a 85% and 95% passing 45 µm product. A significant power saving of 39.2% was observed for the dry VRM compared to the wet ball mill (37.7% for the circuit). The capital investment for the dry Loesche VRM circuit was found to be 10.5% more expensive than that of a wet milling circuit, while the reduced power consumption combined with the decrease in grinding media and wear component consumption resulted in a 5.9% reduction in the operating expenditure (OPEX). The viability of the VRM in this application would also enable off-site dry milling of limestone, presenting significant potential capital and operating cost benefits for a power plant.

KEYWORDS:

• Flue gas desulfurization (FGD)
• vertical roller mill (VRM)
• limestone
• wet milling
• dry milling
• capital expense (CAPEX)
• operational expenditure (OPEX)

Acknowledgments

Cara Swart is grateful to Hatch for their support and financial contribution. The authors record their thanks to Idwala Lime for the supply and transportation of limestone samples, and to Eskom’s Kusile Power Station for information covering the FGD ball milling application. The authors also acknowledge with thanks the testwork performed by Loesche in Germany, and Mintek in Johannesburg for sharing their Comminution Laboratory facilities.

Disclosure statement

No potential conflict of interest was reported by the author(s).