Ann: Presentation at MB 25th Bauxite & Alumina Conference, page-5

Don't blame the presentation. The drop is due to more supply coming online.
News yesterday (the presentation is coincidence)

"Hydro said it had agreed with the Brazilian government for a third party to conduct a technical assessment on two clearances that, if confirmed, could lead to a full ramp-up in output at its Alunorte refinery."
https://finance.nine.com.au/2019/03/28/07/41/aluminium-at-one-week-high-as-stocks-fall