Customs duty exemption limited by prescribed effective rates for Chapter 25 mineral and related imports. The notification fixes effective rates of basic customs duty for goods under Chapter 25, exempting imports only to the extent that leviable duty exceeds the specified ad valorem rates. It sets a general 70% ad valorem rate for most Chapter 25 goods and assigns specific effective rates for listed items: rutile for non-metal extraction use at 70%, insoluble sulphur at 55%, cement at 55%, raw asbestos at 40%, natural cryolite at 55%, and fluorspar at 70%.