Exhibit 99.1

JOINT FILING AGREEMENT

Each of the undersigned hereby agrees that this Amendment No. 20 to the statement on Schedule 13D is being filed with the United States Securities and Exchange Commission on behalf of each of the undersigned pursuant to Rule 13d-1(k) under the United States Securities Exchange Act of 1934, as amended.

Dated: April 4, 2018

Glencore AG
By:	/s/ Thomas Meier
Name:	Thomas Meier
Title:	Officer
By:	/s/ Martin Häring
Name:	Martin Häring
Title:	Director
Glencore International AG
By:	/s/ Aristotelis Mistakidis
Name:	Aristotelis Mistakidis
Title:	Officer
By:	/s/ Martin Häring
Name:	Martin Häring
Title:	Officer
Glencore plc
By:	/s/ John Burton
Name:	John Burton
Title:	Company Secretary