Receipts from selling chemicals or reagents to any mining, milling or oil company for use in processing ores or oil in a mill, smelter or refinery or in acidizing oil wells, and receipts from selling chemicals or reagents in lots in excess of eighteen tons to any hard-rock mining or milling company for use in any combination of extracting, leaching, milling, smelting, refining or processing ore at a mine site, may be deducted from gross receipts. Receipts from selling explosives, blasting powder or dynamite may not be deducted from gross receipts.