I want to delete 20000 records and I don't want them to be recovered from recycle bin. Is this possible if yes then how?

There are various ways to skip the recycle bin,one of them are:

1.Change Properties to Skip Recycle Bin

• Log in to your Windows PC.
• On the Desktop, locate Recycle Bin folder.
• Right-click on the Recycle Bin and choose Properties.
• Under ‘Settings for Selected location’, choose the ‘Don’t move files to Recycle Bin. Remove files immediately when deleted.’ radio button.
• If you want to prevent accidental deletion, click the ‘Display delete confirmation dialog’ checkbox.
• Click the ‘Apply’ button and then ‘OK’ to save the changed Recycle Bin settings/properties.
  From now on, when you delete a file or folder, it will be permanently deleted from the system. Windows will automatically skip the Recycle Bin and based on your settings, you may get prompt to confirm permanent file deletion.

Yes this is possible by Using "Hard Delete" Option in data loader.By using Hard Delete option data will permanently delete from salesforce and it will not go to in Recycle Bin. And also you can not recover deleted records.