Suleiman Kerimov, the metals billionaire and Russian senator, bought 37m shares on 29 April and added a further 1.3m shares a day later.
The purchase makes Kerimov one of ENRC’s biggest shareholders outside of the consortium considering a takeover bid for the FTSE 100 component.
Kerimov’s other investments include the single largest stake in Polyus Gold, a holding in fertiliser group Uralkali and building company PIK.
ENRC’s three founding shareholders, plus the Kazakh government, said last month they were looking to buy out the company.
The group, which already owns more than 55 per cent of ENRC’s shares, has been given until 17 May to make a firm bid.
ENRC is under investigation by the Serious Fraud Office over claims of bribery in Africa and Kazakhstan.
The firm disputes the claims.