Arch Chemicals Inc. (NYSE: ARJ) shares rose Monday by 11.2% to $46.88. The biocides firm agreed to be acquired by Swiss chemical company Lonza Group AG in a cash transaction valued at $1.4 billion, the companies said. Volume for the stock ballooned to more than 8.5 million shares, over a daily average of less than 198,000.

A release dated July 11 stated that the Norwalk, Conn.-based Arch received an offer from Lonza representing a 36.7% premium to Arch Chemicals’ average closing price over the last 30 trading days. Based on the offer price for all the outstanding shares, Arch Chemicals’ enterprise value would be $1.4 billion (approximately 1.25 billion Swiss francs).

Lonza’s cash offer is subject to customary conditions including the tendering of more than two-thirds of Arch Chemicals’ outstanding shares of common stock and clearance from antitrust regulatory authorities. Lonza expects to commence the tender offer by July 15 and to complete the tender offer later in 2011.

Said Arch CEO Michael Campbell in the same release, “We are pleased to have reached this agreement with Lonza, a company that knows our business well and shares our commitment to continuous improvement in innovation, operational excellence, safety and sustainability.

“We are confident that we have found the right strategic partner to help our business reach the next level of success. This compelling transaction offers Arch Chemicals shareholders a meaningful premium for their shares and will create exciting opportunities for Arch Chemicals employees, while enhancing offerings for customers.”

Arch Chemicals, Inc. is a global Biocides company with annual sales of over $1 billion U.S. Arch and its subsidiaries provide innovative, chemistry-based and related solutions to destroy or to selectively inhibit the growth of harmful microorganisms.