LONDON, UK: Rio Tinto (ASX: RIO) has agreed to invest A$40.4 million in Sovereign Metals Limited (ASX: SVM) to acquire an initial 15% stake in the company and support the development of its Kasiya Rutile-Graphite Project in Malawi. The investment will enable Sovereign to advance the Kasiya project, which hosts a large-scale, high-grade, low-cost and…