Hi @Ashok121 , that’s certainly a lot of Excel files to be processing!
You could perform this by wrapping the Excel Reader with try-catch nodes and then handling any errors by then transferring files that wouldn’t open to a separate folder.
Here’s a demo workflow
This may take a while to run with 500k files 
This workflow was written using KNIME 5.1, but should work equally with KNIME 4.7.
The NoOp node is available from NodePit, and does nothing except provide a means of doing what the branch needs to do!
If you can’t/don’t want to install that, replace it with an Add Empty Rows node, with the config “Additional” set to 0, or you can use my “pass through component” which I wrote before discovering “NoOp”