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I think you can use the "Active" key of an existing account to mine. Copy the key pair from your Steemit account. It is under the "Permissions" section.

Perhaps need to become a witness first, by using update_witness command in the CLI (better generate another key and set it as signing key).

Then in config.ini,

witness = "your_account"
miner = ["your_account","your_active_key"]
private-key = your_signing_key

Note: no quotes around the private-key.

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