El grupo minero canadiense Sherritt International anunció el martes que se desprenderá de sus actividades en el sector carbonífero, que venderá en 946 millones de dólares, a fin de concentrarse en el níquel y petróleo.

La operación obligará al grupo a contabilizar en su cuarto trimestre una pérdida de 460 millones de dólares, según un comunicado de la empresa.

Esos activos carboníferos habían sido comprados por Sherritt en 2008 y gran parte de su pérdida de valor registrada en la venta se explica por la necesidad de realizar una depreciación previa de 308 millones.

Sherritt International Corp. S.T +14.79% is facing pressure from an activist shareholder amid plans to sell its coal business and focus on its nickel and oil operations.

Clarke Inc., CKI.T +0.25% a Canadian investment company known for its activism, wants the Toronto-based natural-resources company to reduce its board size to seven from nine and replace three directors with its nominees, Clarke Chief Executive George Armoyan said in an interview Tuesday.

Halifax-based Clarke owns about 5.2% of Sherritt, Mr. Armoyan said. That would make Clarke the company's largest shareholder. Its next largest shareholder is Dimensional Fund Advisors LP, which has a 3.6% stake according to FactSet data.

"We should have representation on the board. We believe we are either the largest or second largest shareholder…and we need to preserve value for all shareholders," Mr. Armoyan said.

Sherritt Chief Executive David Pathe said in an interview that the company received Clarke's letter on Monday and its board hasn't had time to consider the investor's request for board seats. "That will happen in due course and I suspect we will respond more completely in the new year," he said.

Mr. Armoyan said current directors own too little of Sherritt stock to justify their current compensation.

He said he is also disappointed with the company's stock price and Sherritt's operational performance, as highlighted by the loss Sherritt will take on the sale of its coal business for a combined 946 million Canadian dollars ($892 million).

Sherritt, which is Canada's largest producer of coal used to generate power, is selling a portfolio of coal assets that include seven producing mines in Western Canada to U.S.-based Westmoreland Coal Co. WLB +11.20% for about C$465 million.

The sale also includes a stake in an activated carbon plant and a char facility which supplies barbecue-briquette producers. Sherritt will receive C$312 million in cash and the assumption of capital leases worth about C$153 million.

Westmoreland called the deal "a transformational opportunity" that will add annual coal output of about 27 million tons, more than doubling its business. It said the purchase will make it the sixth largest North American coal producer, as measured by 2012 production.

Sherritt also is selling its Canadian coal and potash-royalty business to a group led by Altius Minerals Corp. ALS.T +12.55% for about C$481 million in cash.

Altius, a minerals-royalty company based in St. John's, Newfoundland, will own a 51% stake in the royalties, with a subsidiary of Boston-based Liberty Mutual Insurance and other private parties as business partners.

A decadelong commodity bull run that started to peter out in 2011 encouraged miners to overpay for projects they are now looking to sell—in many cases at a loss—as they try to cut their heavy debt loads and return to profitability amid a slowdown in demand for resources from China and other emerging markets.

Sherritt, whose recent financial results have been pressured by lower commodity prices, said it estimates a loss on the sale of about C$460 million, C$308 million of which would be goodwill related to the purchase of coal assets in 2008.

The mining industry's struggles have also spurred greater scrutiny of the sector by disappointed shareholders.

Earlier this month, Barrick Gold Corp. ABX.T +3.84% , one of the world's biggest gold miners, announced several boardroom changes to ease investor concerns over its corporate governance.

Sherritt's shares rose nearly 15% on Tuesday to C$3.88.

So far this year, the stock is down about 32%, a bigger drop than the 24% decline in Canada's flagship S&P/TSX diversified Metal Index.

Write to Ben Dummett at ben.dummett@wsj.com and Carolyn King at carolyn.m.king@wsj.com