I'm glad you've enjoyed the process. This sort of ship is a bit more of a flying oil rig than something that can extract solid materials in quantity, but it's being used here to take core samples. In more mainstream use, these would be used to tap pockets of liquid hydrocarbons under the surface of the outer moons. While simple hydrocarbons at the bottom of a gravity well would be of marginal use as fuel, they're more useful as precursors to more complex materials. Typically, these would feed into an automated complex that included a cracking tower.
Thanks. For the project this and some of the other designs are going for, I'm pretty much obligated to think these through, or at least look like I am. You see, this and many of the others in my gallery will be part of a book I'm working on concerning what the working boats of the outer planets might look like in perhaps 150 years.
That's something I've always loved about your works, the amount of technical detailing that tends to go into them, rather than "look! It's a spaceship with guns on only one side for no reason other than looking like a flying battleship!". Sorry, rambling a bit, but I hope to be able to build models to a similar level that you draw some day.