Sodium nitrite is hygroscopic an may therefore form pretty hard clumps.
But these Clumps can easily be pounded in a mortar. The useful compound is each time separated by sieving. To get 1 kg pourable again 15 minutes are needed, wherat the sieving is the key for success. Use a simple halfround household wire sieve. Moisture may be removed on heating in vacuum at 170 °C.
Dispose in a separate bottle.