All elements in rock phosphate except PO43- or Ca2+ are considered as impurities. Major impurities are MgO, Fe2O3, Al2O3, SiO2, Na2O, K2O, fluoride, chloride, Uranium, Cadmium, Insoluble, Sulfate, and Carbonate etc. Some of these impurities are soluble in phosphoric acid. Insoluble impurities do not have major impact on phosphoric acid manufacturing process as these are separated out with gypsum during filtration. All problems are created by the soluble impurities. Problems such as scaling, corrosion, production of sludge, increase of acid viscosity and process instability occur due to the presence of these impurities in the acid. Such problem can be caused by either one single ion or the presence of a complex group. For example a small increase of the potassium, aluminum or iron content of phosphoric acid can produce a large quantity of sludge in the storage tanks. Sometimes impurities like silica and aluminum are added to the reactor to improve the filterability of gypsum and reduction corrosion due to fixing the fluorine as SiF4. Presence of high chlorides causes severe increase in corrosion rates. Combined with fluoride, sodium or potassium, silica can form fluosilicate salts which cause heavy scaling on filter cloth, pan, and piping while reducing filtration rates. Some of the major impurities are:
The main significance of CaO is that it consumes sulfuric acid by the following reaction.
CaO + H2SO4 = CaSO4 + H2O
The quality of rock is assessed by viewing the ratio of CaO/P2O5 rather than just the CaO content. When reviewing at the effect of CaO, Al2O3 and MgO in the rock there are clear relationships between the CaO/P2O5 and the BPL, Aluminum and MgO content of the rock. The CaO/P2O5 can be reduced by 0.05 by an increase of 9 units of BPL, a decrease of 0.40% MgO or an increase of 0.25% Al2O3.
Magnesium is present in the rock mainly as Dolomite (mixture of calcium and magnesium carbonate). Essentially all the Mg dissolves in the digestion of the rock. Since all the P2O5 in the rock does not dissolve, it is possible to have a higher ratio of MgO/P2O5 in the digestion acid than in the rock. MgO is known to have a significant effect on P2O5 recovery in the acid digestion step. Based on operating data, an increase of 0.1% in MgO in the rock will decrease P2O5 recovery by at least 0.25%. MgO can also affect the production rate. There is a 4.3% reduction in rate for every 0.1% increase in MgO.
Fluorine is removed mainly due to evaporation of phosphoric acid to a higher P2O5 strength. Evaporating the acid (particularly to 54% P2O5) liberates the fluoride as SiF4 gas which is absorbed in scrubber as fluosilicic acid (up to concentration of 18%). Some quantity of fluorine is also removed in the attack section of the reaction tank. This removal can be enhanced by using the high heat of dilution of the sulfuric acid during addition. Most of the fluorine that is removed has to be replaced with sulfate or nitrate ions to keep the anion/cation balance in the slurry. Fluoride like chloride rapidly increases corrosion rates and requires special protection.
Iron is mainly removed from the phosphoric acid by desludging during concentration. It precipitates when the acid is evaporated. The precipitated iron can be removed by centrifugation or clarification. The iron precipitates as complex iron phosphates and it is economically desirable to utilize them in other fertilizer products. Unlike MgO, which does not significantly precipitate upon concentration of the acid, essentially all iron above 0.9% in the rock will precipitate as the acid is concentrated.
Most of the phosphate rocks contain small quantities of uranium. The quantity is so small (about 1 pound per ton of P2O5 for Florida rock, generally less for other phosphate rocks) that it has an insignificant impact on the ability to make fertilizer grade acid.
Cadmium being a heavy metal is a major concern in Europe and other areas of the world as a residue in fertilizers. This is because cadmium is taken up by some plants and passed through the food chain. Many of the world’s phosphate rocks contain 10’s of PPM of cadmium.
All phosphates contain some sulfate. This corresponds to free sulfate and is advantageous when calculating the sulfuric acid required digesting the phosphate rock. However, it is relatively constant at about 0.8% in most phosphate rocks.
Carbonate is contained in all phosphate rocks. It is present mainly as limestone or dolomite form. Presence of high carbonate causes excess foaming in the rock digestion process and can cause increased phosphate losses due to the foaming and overflowing. Generally the foaming is controlled by the use of a defoaming agent. The quantity of carbonate in phosphate rocks should correlates well with the MgO content of the rock.
The importance of Insoluble:
Insoluble is the material remaining after digestion of the rock. In general the digestion leaves mostly the silica as the insoluble. In the plant, a significant portion of the silica dissolves in the acid forming fluosilicic acid. On the other hand some species such as iron pyrites are not soluble in the plant digestion.
Low grade rock limits the production rate and P2O5 recovery in the phosphoric acid manufacturing process. Processing low grade rock phosphate ores is a challenge that will require increased attention in the decades to come as the overall quality of rock ores is depleting.