If you change the primary use of an item
If you buy an item exempt to use in a mining activity, it becomes taxable if you stop using it in a mining activity. You must then pay tax on the fair market value of the item.
Example
You buy a backhoe that you’ll use strictly in moving ore from the mine. The backhoe qualifies for the exemption because it’s used directly and primarily in a necessary activity. After one year, you replace the backhoe with a new model, and use the old backhoe strictly to reclaim land. Reclaiming land isn’t an exempt activity, so the backhoe is now taxable on its fair market value. You must determine the fair market value of the backhoe, and pay use tax on that amount. All parts and supplies to repair and maintain the backhoe from that point on are also taxable.
Note: The opposite isn’t true. If you pay tax on an item because you use it in a non-mining activity, you can’t claim a tax credit if you begin to use it in a mining activity.