The invention relates to a method of controlling the solubility of granular material by treating a precipitate comprising iron(II)sulphate monohydrate originating from titanium dioxide production, in which method the reaction temperature is allowed to increase at the most to a temperature of 120° C., to a plant for treating a precipitate comprising iron(II)sulphate monohydrate, to a product and its uses. A typical method according to the invention comprises the steps of mixing water to the reaction mixture in the said mixing apparatus, and keeping the amount of neutralizing agent sufficient to give a pH value from 1.5 to 4, preferably from 1.5 to 3, to the end product in order to obtain a granular material having a high soluble iron(II) content or keeping the amount of neutralizing agent sufficient to give a pH value ≧9 to the end product in order to obtain a granular material having a low solubility.