BEIJING Feb 8, 2010 Chinese inspectors have shut dairy firms in the northwest after they found contaminated milk powder there. They seized 72 tonnes of milk powder tainted with melamine. Melamine is a compound commonly used in plastics or fertilizer. But it can also be added to foods to show high protein levels in tests. The firms closed are Tiantian Dairy Co Ltd in Ningxia province, which was found to have sold 170 tonnes of  the tainted milk powder, and Ningxia Panda.  Ningxia Panda was shut because of its ties to Shanghai Panda Dairy Co, which was closed late last year for selling products tainted with melamine. Nearly 100 tonnes of tainted milk powder may still be on shop shelves, the China Daily reported on Monday. In 2008, a scandal which damaged the reputation of the Chinese dairy industry had erupted. At least six children were killed and 300,000 needed treatment after drinking contaminated milk.   A number of cases of melamine in milk have surfaced in the past few months, some of which appear to have come from old batches of contaminated powder that was never destroyed. There have been no reported deaths or illnesses from the latest batches of tainted milk.