Actually, the Peaceable Army (or Armies) of Queens problem is a separate problem and much harder than the n-queens problem. Here is a MiniZinc model: http://hakank.org/minizinc/peaceableArmyOfQueens.mzn , and a Picat model: http://hakank.org/picat/peaceableArmyOfQueens.pi

(Also: Thanks for your links to my site.)