The impact of ore impurities on the intrinsic quality and consumption of phosphate ammonium (in more detail)

Phosphorus ore certainly plays a decisive role in the production of phosphoric acid: natural phosphates are used as raw materials for manufacturing phosphate fertilizers. In industry, phosphate deposits with industrial mining value are collectively referred to as phosphate rocks. Natural phosphate rocks are mainly apatite and phosphorite, whose main chemical component is calcium fluorophosphate. Its chemical formula is Ca5F(PO4)3. In fact, calcium fluorophosphate is a complex compound formed from three molecules of calcium orthophosphate and one molecule of calcium fluoride; its molecular formula should be 3Ca3F(PO4)2•CaF2, or it can also be written as Ca10F2(PO4)6. Ca5F(PO4)3 is the commonly used shorthand form. (1) Purity of phosphate rock: The purity of phosphate rock refers to the content of P2O5 in it. In our country, it is customary to express it as a percentage of P2O5. Internationally, it is commonly expressed in terms of BPL, which means converting the P2O5 content of phosphate rock into an equivalent value for calcium triphosphate, Ca3(PO4)2. The theoretical P2O5 content of calcium triphosphate is 45.76%; therefore, when phosphate rock contains 0.4576% P2O5, it is indicated as 1% BPL. That is: %BPL×0.4576=% P2O5. High-grade ores generally contain more than 30% P2O5, while medium-grade ores contain 26%-30% P2O5 ； Ore with a P2O5 content of less than 26% is called low-grade ore. In the production of wet-process phosphoric acid, the P2O5 content of phosphate rock primarily affects economic efficiency. The lower the grade, the lower the economic efficiency per unit mass of P2O5 produced; for example, the volume utilization factor of the reaction tank and the production capacity of the filter will decrease. In phosphate production, the P2O5 content of phosphate rock is another important factor determining the system’s water balance, when the concentration of phosphate produced remains constant. The lower the grade of phosphate ore, the less water amount allowed to be added to the filtration system according to material balance calculations, and this affects the degree of washing of phosphoric acid. When the amount of water used for washing is reduced to such an extent that it is no longer sufficient to remove the free acids in the filter residue, it becomes necessary to lower the concentration of phosphoric acid produced. This, obviously, will result in a decrease in the capacity of the equipment used for concentrating the slurry or for producing phosphoric acid. It can be seen that the higher the grade of phosphate rock, the greater the production capacity of the equipment, and the better the product quality and economic benefits. This is the main reason for advocating a policy of using high-quality raw material phosphate rock. (II) Content of harmful impurities in phosphate rock: Phosphate rock contains various impurities, and these impurities generally increase acid consumption during the production of wet-process phosphoric acid and acid-based phosphate fertilizers. They reduce product quality and increase production costs, as well as diminish the manufacturing capacity of production facilities. Moreover, they accelerate corrosion and wear of equipment materials, thereby reducing the operational efficiency of the equipment. In the production of wet-process phosphoric acid, if the content of harmful impurities is too high, it can prevent the reaction process involving phosphate rock and the crystallization process of calcium sulfate from proceeding properly; in some cases, it may even be impossible to produce phosphoric acid at all. Even if phosphoric acid is produced, it cannot be concentrated or processed for use due to excessive impurities. Phosphorus ores contain many impurities, but the ones that have the greatest impact are usually iron, aluminum, and magnesium; followed by carbonates, organic substances, dispersed silts, chlorine, etc. 1. The CaO content in phosphate rock (referring to the CaO/P2O5 ratio) is a key factor determining the amount of sulfuric acid consumed in the production of wet phosphoric acid. The CaO/P2O5 ratio determines the amount of sulfuric acid required to produce a unit mass of P2O5. With a constant P2O5 content in phosphate ore, the higher the CaO content, the greater the amount of sulfuric acid required (1.75 parts of sulfuric acid are needed for each part of CaO). At the same time, as the CaO content increases and the gypsum value rises, the filtration load increases accordingly, resulting in a decrease in the P2O5 production capacity per unit area of filtration equipment. Therefore, the CaO/P2O5 ratio is required to be close to that in pure fluorapatite Ca5F(PO4)3. The theoretical ratio of CaO to P2O5, with a mass ratio of 1.31 and a molar ratio of 3.33, should not be exceeded too much; exceeding this value requires the use of additional sulfuric acid. 2. The content of sesquioxides R2O3 in phosphate rocks: Sesquioxides refer to the amounts of iron and aluminum oxides present in phosphate rocks, and are usually expressed as R2O3 (where R represents Fe and Al, that is, Fe2O3 + Al2O3). In the production of wet-process phosphoric acid, iron and aluminum not only interfere with the growth of calcium sulfate crystals but also cause phosphoric acid to form sludge, a problem that is particularly severe in concentrated phosphoric acid. Its precipitation or removal along with gypsum will result in significant loss of P2O5. The formation of fine crystals of iron and aluminum complex phosphates not only increases the viscosity of the solution and slurry but also easily clogs the filter cloth and the pores in the filter cake, thereby reducing the filtration efficiency and the production capacity of the equipment. Iron and aluminum impurities also often precipitate during phosphate production, forming scale ； Sediment precipitates during storage and transportation, causing difficulties in these processes. Phosphates of iron and aluminum also pose difficulties in subsequent processing such as the concentration and drying of phosphoric acid or ammonium phosphate slurries, leading to poor product properties and a decline in quality. 3. Magnesium oxide content in phosphate rock: The magnesium salts in phosphate rock (expressed as MgO) generally dissolve completely as a result of reactions and end up in the phosphoric acid; they do not tend to precipitate even after concentration, this being due to the high solubility of magnesium phosphates in phosphoric acid solutions. This is also the reason why magnesium salts have severe adverse effects. The highly acidic Mg(H2PO4)2 significantly increases the viscosity of phosphoric acid, resulting in difficult ion diffusion during the acidolysis process and uneven local concentrations. This hinders the uniform growth of calcium sulfate crystals and makes filtration more difficult. During the acidolysis of phosphate rock, the presence of Mg partially neutralizes the first hydrogen ion in phosphoric acid, reducing the concentration of hydrogen ions in the solution and severely affecting the reactivity of the phosphate rock. If the amount of sulfuric acid is increased to maintain a certain H+ ion concentration, it will lead to an excessively high concentration of SO42- in the solution; this not only increases sulfuric acid consumption but also makes it difficult for calcium sulfate to crystallize. Furthermore, since magnesium salts also produce a certain amount of water-soluble phosphates during the reaction process, and their impact on the hygroscopicity of the product is greater than that of iron and aluminum salts, this affects the physical properties of the product, reduces its water solubility, and leads to a decline in quality. The high solubility of magnesium salts also significantly increases the viscosity of phosphoric acid, having a very adverse effect on subsequent processing steps such as phosphoric acid concentration or slurry concentration. 4. Content of silicon and acid-insoluble substances: Phosphorus ores always contain varying amounts of silicon, which exists in the form of acid-insoluble substances primarily as SiO2. SiO2 does not consume sulfuric acid during the reaction, and part of the SiO2 can also convert the highly toxic gas HF into SiF4, a gas with lower toxicity, thereby reducing harm and corrosion. During the reaction process, the SiO2 portion is eroded by the HF generated as a byproduct of the reaction. The degree of erosion is related to the physical properties of silicon; silicon with higher reactivity readily reacts to produce hydrofluoric acid (HF) from which fluorosilicic acid (H2SiF6) is formed. The corrosivity of fluorosilicic acid toward metal materials is much lower than that of hydrofluoric acid. To this end, the phosphate rock should contain the necessary SiO2; when SiO2/F is less than the stoichiometric value, soluble silicon should also be added. However, excessive SiO2 is harmful; on the one hand, the colloidal silicic acid present in wet phosphoric acid affects the filtration and separation of phosphogypsum ； On the other hand, it increases the hardness of phosphate rock, reduces the production capacity of mills, and increases equipment wear. Small acid-insoluble particles (-320 mesh, i.e., particles of 44 microns or less), also known as tripartite argillaceous substances, can cause significant harm to the production of phosphoric acid and phosphate fertilizers. In phosphorus ore that has undergone enrichment treatment, the mud is always removed first; therefore, its impact is minimal. However, when using untreated raw ore in production, the influence of clay becomes more apparent. Most phosphorus ammonium processes using the slurry method use raw ore directly, so due attention and care should be paid to this. The fine particulate clays in phosphate rocks are not decomposed by sulfuric acid during the reaction process; they precipitate along with phosphogypsum. During the filtration of the slurry, these particles easily clog the pores of the filter cloth, **reducing the filtration speed and shortening the service life of the filter cloth, thereby causing serious difficulties in the processing process. Even a clay content of only 4%-5% in phosphate ore can have a significant impact on filtration. It is evident that the hazard of dispersed clay in phosphate rocks cannot be ignored; phosphate rocks with high clay content should preferably undergo washing to remove the clay first. 5. Content of organic matter and phosphates: Most phosphate rocks, especially sedimentary phosphate rocks, often contain organic matter. A high content of organic matter can cause a lot of **troubles** in operation. Carbonates and organic substances cause bubbles to form during the reaction process; organic substances also help to turn the CO2 gas produced by the reaction into stable foam, a phenomenon commonly known as foaming. Foam reduces the effective volume of the acidolysis tank, and it also causes difficulties in the reaction of phosphate rock, as well as in the transportation and filtration of the slurry. Furthermore, organic matter forms extremely fine carbon particles as a result of carbonization, which can easily clog the filter fabric, reduce the porosity of the filter cake, and thereby decrease the filtration strength. Organic substances can also affect the color of the product’s acid. Therefore, the foam phenomenon is also one of the indicators for evaluating the properties of phosphate rocks. 6. Other components: Fluorine is a major component of phosphate rocks, and it usually exists in a certain proportion to the P2O5 content; therefore, the fluorine content in phosphate rocks is generally not used as an indicator for evaluation. But attention should be paid to the chlorine content in phosphate rocks. This is because chlorine causes more severe corrosion than fluorine.

There’s still no answer regarding the change in quality...

Theoretically, high-grade phosphate ores can indeed be used to produce phosphatic ammonium products with high nutrient content. However, since each type of ore contains different impurities, which in turn affect the amount that dissolves in acid, and since the concentration of SO3 during the extraction process varies, as does the quality of the acid (such as its solid content), all these factors have an impact on the nutrient content of the resulting products. In recent years, our factory has faced many such problems; of course, we have tried various ways to address them. If summarized, these efforts could form a quite good article, but it cannot be published casually.