The first phase of a planned 116-unit apartment complex has opened on the site of the old Brigham’s Ice Cream factory on Mill Street. Wood Partners LLC announced that the first 36 apartments of the Alta Brigham Square complex near Arlington Center have opened, and the remaining 80 units are on track to open next month. Work on the project began last fall, when the former factory was demolished. The site had been idle since Brigham’s stopped making ice cream and sold its assets in 2008. The Georgia-based Wood Partners is building a four-story, 135,000-square-foot complex that will include 3,500 square feet of ground-level retail space.