You can accomplish this by setting the GNUPGHOME environmental variable to another directory, then receive keys to the alt keyring in it. None of the gpg actions you perform in the context of this alternate gnupg home will effect the keyring or any other data in your normal gnupg home.
The GNUPGHOME you set will remain in effect only for this terminal session. When you close the terminal window the gnupg home directory will revert to the default ~/.gnupg. You can either create a persistent directory to use for this or just create a temporary directory on the fly:
$ export GNUPGHOME=$(mktemp -d)
Now retrieve the key:
$ gpg --keyserver pool.sks-keyservers.net --recv-keys 648ACFD622F3D138
Now you can display the info for the imported key:
$ gpg -k 648ACFD622F3D138
And export the ascii-armored key file to your home directory.
$ gpg -ao ~/648ACFD622F3D138.asc --export 648ACFD622F3D138
Just close the terminal window when you're done using the temporary home directory. If you decide you want the key in your keyring, import it from the file you exported.