“Also, you generally don’t mix it with other compounds until you are ready to use it.”
They mixed it at the plant and took it to the mine in containers to pour it into the blast holes. The plant was no bigger than a common sized machine shed.
I don't have first hand experience in actually seeing them mix it like you do but from the satellite photos, it looks like that little ol machine shed was about 700 feet long and about 250 feet wide. I wish I had a machine shed that size.