Minerals Technologies Inc. boosted its bid for AMCOL International Corp. to $45 a share, topping a recently sweetened bid from Imerys and raising the stakes in a bidding war over the U.S. bentonite producer.

AMCOL, which posted sales of more than $1 billion in 2013, is the world's largest producer of bentonite, a mineral used in many industries such as car manufacturing and heavy machinery.

AMCOL shares rose in trading Monday, closing at $46.35 in a sign investors expect the bidding war to continue.

AMCOL confirmed it received Minerals Technologies' revised proposal and said that its board has determined that the bid could lead to a superior proposal, but continues to recommend the transaction with Imerys to its stockholders at this time.

Imerys couldn't immediately be reached to comment.

Minerals Technologies claimed its revised proposal represents a superior offer to Imerys's recently sweetened takeover bid on Wednesday of $42.75 a share. The latest bid from Minerals Technologies also represents an increase of $2.50 a share from its own boosted offer of $42.50 a week ago.

Minerals Technologies Chairman and Chief Executive Joseph C. Muscari said Monday that "the merits of strategic fit, a substantial portfolio of future growth opportunities, and significant readily available synergies present a compelling opportunity for our shareholders, and, at the same time, our offer delivers the greatest value available to AMCOL's shareholders."

Founded in 1927, AMCOL, based in Hoffman Estates, Ill., makes products that are used in a wide variety of items including face moisturizers and cat-box filler, and for oil-and-gas drilling.

Anna Prior contributed to this article.

Write to Tess Stynes at tess.stynes@wsj.com

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