Phosphoric acid is produced from phosphate rock by reacting it with concentrated sulphuric acid in a series of reactor vessels. This results in phosphoric acid and calcium sulphate (gypsum) together with smaller volumes of impurities from the mined rock. Water is added and the gypsum is removed by filtration along with other insoluble materials (e.g., silica). The product from the ‘wet process’ acid is impure but can be used in the fertilizer manufacture process. Alternatively, it can be concentrated to create a liquid fertilizer. Typically, this impure acid is purified further when producing phosphates at a large scale to ensure undesirable elements (i.e., heavy metals) are removed.