Private key from seed

I have waves full node on my server.

Using REST API I’ve generated a few addresses(like POST /addresses). With REST API I can get encoded seed for each of this address, for example

GET /addresses/seed/3PNGoXRgSVPum5yeBVt9oK6fmnW7r7hPbT3

  "address" : "3PNGoXRgSVPum5yeBVt9oK6fmnW7r7hPbT3",
  "seed" : "9cD6i4HGRvY3fJXBx76BbH8WiDEfYCgpC938hJFP4B2a"


But to send money from those addresses I need a corresponding private key.
So how can i get it?

Or how can I send money from those addresses?