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Explanation: Sorry to have to rain on the other answerers' parades, but this is actually about economics, not about chemical or process engineering (or even materials management).
"Rohstoffe" in economics is very often "commodities" in English, and that's generally the case here as well. Plenty of solid Google hits for "commodity flows", including together with "dynamic model(l)ing".
"zum Beispiel das weltweite Kupfermodell und dessen Massenströme"
Mass flow would normally be referring to experimental weights or physical weights of materials consumed, but not as RAW materials; material flow to the mining/recovery/extraction/processing and transport of the materials to their consumers. So I'm totally mystified why the term "-ruckgewinnung" has been thrown in as an excuse for disagreeing with heart's posting.