pixel_player20298493 did you watch the video?. About 35 seconds in, it begins to show the process of how/where to convert the granite. Here's a written explanation as well. :
You don't get the keys by simply completing the mines, per se. You have to convert your granite to Scarab Coins.
You get a key for each level of the Pyramid Builder Achievement you complete. You must convert your granite to coins for the Achievement. Go to your Achievements section & click on the cup for this event. It will tell you how many coins you need for your current level.
1 → 45 coins
2 → 50 coins
3 → 55 coins
4 → 60 coins
5 → 66 coins
6 → 73 coins
7 → 81 coins
8 → 90 coins
PLEASE NOTE: Once you convert your granite to coins, you will have to go to your ACHIEVEMENTS section to collect the rewards for each completed coin level in order to get your keys.
CONVERTING GRANITE TO COINS
Click on the Blue Globe icon on the right side of your camp screen. Then click on the brown bar (button) that says 10 granite → 10 coins to complete the conversion.
This is how the globe icon appears in mobile:
Computer version, has an icon featuring a pyramid.