Richards Bay Minerals (RBM), a Rio Tinto Plc (<a href="https://www.londonstockexchange.com/stock/RIO/rio-tinto-plc/company-page" target="_blank">LON:RIO</a>) company, said on Thursday that it has signed a 20-year power purchase agreement (PPA) to off-take electricity generated by a new 140-MW wind farm in South Africa.