Exhibit 3.15

State of Delaware

Secretary of State

Division of Corporations

Delivered 12:18 PM 10/22/2012

FILED 12:01 PM 10/22/2012

SRV 121150763 - 5230828 FILE

CERTIFICATE OF FORMATION

of

EAGLE NATRIUM LLC

This Certificate of Formation of Eagle Natrium LLC (the “LLC”), dated October 22, 2012, is being duly executed and filed by Lauren D. Gojkovich, as an authorized person, to form a limited liability company under the Delaware Limited Liability Company Act (6 Del.C. § 18-101. et seq.)

FIRST, The name of the limited liability formed hereby is:

Eagle Natrium LLC

SECOND, The address of the registered office of the LLC in the State of Delaware is c/o Corporation Service Company, 2711 Centerville Road, Suite 400 in the City of Wilmington, County of New Castle, State of Delaware 19808.

THIRD, The name and address of the registered agent for service of process on the LLC in the State of Delaware is Corporation Service Company, 2711 Centerville Road, Suite 400 in the City of Wilmington, County of New Castle, State of Delaware 19808.

IN WITNESS WHEREOF, the undersigned has executed this Certificate of Formation as of the date first above written.

/s/ Lauren D. Gojkovich

Lauren D. Gojkovich, Authorized Person