By-Product Potential at York Potash Project

For immediate release: 0700hrs, 16 February 2011

Sirius Minerals Plc

("Sirius" or the "Company")

By-Product Potential at the York Potash Project

Further to the announcement of 14 February 2011 in which the Company stated it was analysing the available process routes and the potential value of associated by-products, the Directors of Sirius Minerals Plc (AIM: SXX, OTCQX: SRUXY), the globally diversified potash development group, are pleased to update the market on potential by-product opportunities at its York Potash Project.

One of the key process routes for polyhalite involves the use of anhydrous ammonia to chemically precipitate potassium sulphate. The key by-product produced in that process is Magnesium Hydroxide Mg(OH)2.

The market for Magnesium Hydroxide is developing at a significant rate. In the last five years Magnesium Hydroxide is increasingly being seen as a viable, and in some cases superior, more environmentally sensitive replacement for Caustic Soda (NaOH) within several significant industries. Its key uses include:

·; Bleaching (Paper Milling): When used in paper making, Mg(OH)2 provides better outcomes (brightness maintained during processing, less pulp loss, cleaner waste water) and reduces the requirements for other bleaching chemicals. A number of paper mills have converted their processes to completely replace caustic soda with Mg(OH)2;

·; Acid Neutralisation (Water Treatment):When Mg(OH)2 is used for water treatment in place of NaOH, roughly ¾ the amount is needed to achieve the same acid neutralisation effect. Furthermore, if Mg(OH)2 is over-applied the pH will not rise much beyond 8 - which is the pH of seawater. If caustic soda is over applied the pH can rise to dangerous levels (12-14); and

·; Fire Suppression: Mg(OH)2 is an excellent smoke suppressant and flame retardant. At temperatures of over 330° C it decomposes to Magnesium Oxide (MgO) and water. This reaction is endothermic - meaning it absorbs heat, which delays or prevents ignition of flammable substances. The water released also dilutes gases (including oxygen) so also assists in the fire suppression.

Based on the chemical composition of polyhalite and the process chemistry, up to 0.33 tonnes of Magnesium Hydroxide could be produced for each tonne of Sulphate of Potash (SOP). Initial market investigations suggest that Magnesium Hydroxide is currently selling for approximately US$500 per tonne. For the York Potash Project, this pricing could potentially contribute a significant by-product credit to reduce the operating costs of the project.

Significant work is needed to confirm the full extent of the potential that Magnesium Hydroxide represents and this will be carried out as part of the Project development studies and will include:

·; Detailed process design and metallurgical and chemical testwork to determine the quantum and availability of Magnesium Hydroxide in process and the suitability of that for sale;

·; Detailed market studies on the structure of the Magnesium Hydroxide and Caustic Soda markets to understand the potential impacts of a major long-term reliable supplier of Magnesium Hydroxide; and

·; Detailed engineering and design of the York Potash Project to maximise the potential value from potash and potential by-product revenue streams.

Chris Fraser, Managing Director of Sirius, commented:

"We already believe we have one of the world's largest deposits of polyhalite. Now, although it is early days, this exciting development could have a significant positive impact on the potential of the York Potash Project. It also further emphasises Sirius' focus to sustainability and looking to better cleaner ways of doing things."

For further information, please visit the Company's website www.siriusminerals.com or the York Potash Project's website at www.yorkpotash.co.uk. Alternatively please contact:

About Sirius Minerals Plc

Sirius Minerals is a globally diversified potash development company. Its primary focus is to bring on stream major potash mining facilities through the acquisition and development of projects overlying recognised potash deposits. Today it holds properties in the United Kingdom (Yorkshire), North America (North Dakota), and Australia (Queensland and Western Australia).

The Company is additionally progressing ongoing Research and Development initiatives into the secondary uses of salt and potash beds for energy storage and carbon dioxide sequestration.

Incorporated in 2003, Sirius Minerals' shares are traded on the London Stock Exchange's AIM market. Its shares are also traded in North America on the OTCQX through the use of a sponsored ADR facility. Further information on the Company can be found at www.siriusminerals.com.

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