A method of treating solid containing material derived from effluent or sludge from a plant for de-inking paper, the material containing calcium in the form of one or more insoluble calcium compounds, the method including the steps of treating the material with an acid to cause dissolution of the calcium thereby forming a calcium ion-containing solution in which insoluble solids are suspended, separating the solution from the insoluble solids and incinerating the separated solids. The solution containing calcium ions may be treated by adding one or more reagents to form a calcium compound precipitate, eg calcium carbonate. The particulate solids produced following the incineration step and following the precipitate formation may be employed as pigments or fillers in paper making or paper coating.