Phosphate grade

First, phosphorus ore grade

The grade of phosphate rock refers to the content of P2O5 in phosphate rock. China is customarily expressed in terms of P2O5 percentage, while internationally, it is expressed in terms of BPL content. BPL is expressed by converting the P2O5 content of phosphate rock into the content of tricalcium phosphate (Ca3(PO4)2). The theoretical content of P2O5 in tricalcium phosphate is 45.76%, so when phosphorus mineralization contains 0.4576% P2O5, it is expressed as 1% BPL.

The calculation method is: %BPL×0.4576=% P2O5 or %BPL=2.1853×=% P2O5

The high-grade rich ore is decreasing with large-scale mining. At present, the requirement for the grade of phosphate rock is: the phosphate ore grade is greater than 31.11-32.03% P2O5 can be utilized under the premise that the impurity content of phosphate rock meets the requirements. The grades of existing phosphate ore in China are generally divided into high grade ore (rich ore: P2O5 content is above 30%), medium grade ore (P2O5 content between 26%-30%) and grade grade ore (poor ore: P2O5 content) Less than 26%).

The production of wet-process phosphoric acid always hopes to improve the grade of phosphate rock in the production process. The grade of phosphate rock directly affects the economic benefits of the plant. The lower the P2O5 of phosphate rock, the lower the economic benefit of producing unit mass P2O5, which is mainly reflected in the volume utilization coefficient of the reaction tank, the decrease of the production intensity of the filter, the increase of the power and consumption index of the equipment, and finally the output of the plant. reduce. As shown in Figure 1, a foreign wet-process phosphate plant, when the production capacity is 150 tons per hour, the relationship between the grade of phosphate ore used and the production of phosphoric acid, it can be seen that when using low-grade phosphate rock, the factory output is obvious. decline.

In the production of phosphoric acid, the phosphate grade is an important factor affecting the production process conditions. When the concentration of phosphoric acid produced is constant, the lower the grade of phosphate rock, the less the amount of washing water allowed to be added to the filtration system according to the material balance, the degree of washing of the filter residue will be affected, resulting in the loss of P2O5 in the gypsum , it is necessary to adjust the phosphoric acid. Concentration, causing fluctuations in the control of the process bar. Table 1 Chemical composition of major phosphorus rock minerals in China

Phosphate contains many impurities, such as iron, aluminum, magnesium, manganese and other metal ions, also contain some radioactive elements uranium, thorium very small amount of rare earth metal lanthanum, ytterbium compound, there are the carbonate ion, silicon Acid salts (or SiO2), fluorine radicals (sometimes all or part of fluorine is replaced by chlorine or acid radicals), sulfates and organics. These impurities, in the wet process phosphoric acid and phosphate industries, generally increase acid consumption, reduce product quality and increase product cost, and also reduce production equipment production capacity, corrosion or abrasion of equipment materials, and reduce equipment driving rate. In the production of wet-process phosphoric acid, if there are many harmful impurities, the reaction process of phosphate rock and the crystallization process of calcium sulfate cannot be performed normally, and even phosphoric acid may not be produced at all. Even if phosphoric acid is produced, it is too concentrated to be concentrated or processed.

Although there are many impurities in phosphate rock, the most important ones are iron, aluminum and magnesium, followed by carbonate, organic matter, dispersible mud and chlorine.

1. CaO content in phosphate rock (referred to as CaO/P2O5)

The CaO content in phosphate rock is the key to determining the consumption of sulfuric acid in the industrial production of wet process phosphoric acid. The CaO/P2O5 ratio determines the amount of sulfuric acid consumed per unit mass of P2O5. In the case of a certain P2O5 content of phosphate rock, the higher the CaO content, the greater the consumption of sulfuric acid (one CaO consumes 1.75 parts of sulfuric acid). At the same time, the CaO content increases, the gypsum value increases, the filtration load increases accordingly, and the P2O5 production capacity per unit area of ​​the filtration equipment decreases. Therefore, it is required that CaO/P2O5 is close to the theoretical ratio of CaO/P2O5 in pure fluoroapatite Ca5F(PO4)3, and its mass ratio is 1.31, and the molar ratio is 3.33, which should not exceed too much. Therefore, exceeding this value requires additional consumption. Sulfuric acid, which adds extra production costs, is therefore a very important technical and economic issue. The ratio of CaO/P2O5 in phosphate rock in China is relatively high, mainly due to the fact that carbonate minerals are often associated with carbonate epiphytic minerals such as dolomite and limestone , which are difficult to remove by general mineral processing methods. This is an urgent problem to be solved in the production of wet-process phosphoric acid.

2. The content of sesquioxide R2O3 in phosphate rock

Perennial oxide refers to the content of iron and aluminum oxide in phosphate rock, which is usually represented by R2O3 (R stands for Fe and Al, ie Fe2O3+Al2O3). Iron and aluminum are mainly derived from clay , and most of them can be removed by screening and magnetic separation. In the production of wet-process phosphoric acid, iron and aluminum not only interfere with the growth of calcium sulfate crystals, but also cause the formation of antimony phosphate, especially in concentrated phosphoric acid. Precipitation or discharge with gypsum will cause a large loss of P2O5. The complex phosphate crystals that form iron or aluminum are small, which not only increase the viscosity of the solution and slurry, but also easily block the filter cloth and filter cake pores, thereby reducing the filtration strength and the productivity of the equipment. Iron and aluminum impurities are also often precipitated in the production of phosphoric acid, forming scale, which precipitates sludge during storage and transportation, which causes difficulties in storage and transportation. Iron and aluminum phosphates also cause difficulties in subsequent processing such as phosphoric acid concentration, resulting in poor physical properties and reduced quality.

3. MgO content in phosphate rock

The magnesium salt of phosphate rock (expressed as MgO) is generally dissolved and present in phosphoric acid after reaction, and is not easily precipitated after concentration. This is because magnesium phosphate has a large solubility in phosphoric acid solution, and it is also a serious adverse effect of magnesium salt. s reason. Mg(H2PO4)2 with extremely high dispersion makes the viscosity of phosphoric acid increase sharply, which makes ion diffusion difficult and local concentration inconsistent during acid hydrolysis, affecting the uniform growth of calcium sulfate crystal and increasing filtration difficulty. In the acid hydrolysis process of phosphate rock, the presence of magnesium causes the first hydrogen ion in the phosphoric acid to be partially neutralized, which will lower the concentration of hydrogen ions in the solution and seriously affect the reaction capacity of the phosphate rock. If the amount of sulfuric acid is increased in order to maintain a certain hydrogen ion concentration, an excessive SO42-concentration will occur in the solution, which not only increases the consumption of sulfuric acid but also causes difficulty in crystallizing calcium sulfate. In addition, since the magnesium salt also generates a part of the bismuth soluble phosphate during the reaction, and the magnesium salt has a greater influence on the hygroscopicity of the product than the iron and aluminum salts, the physical properties of the product are affected, and the water solubility is lowered. decline in quality.

The MgO content in phosphate rock has become one of the main indicators for evaluating the quality of phosphate rock. The requirements for MgO content in foreign production plants are very strict. The content of MgO in phosphate rock in China is obviously high, which has adverse effects on the production of phosphoric acid and the production of phosphate. Therefore, research on methods to reduce MgO content has become an important topic in the research and development of phosphate rock production in China.

At present, magnesium reduction in phosphate rock has two processes of physical beneficiation and chemical magnesium. The disadvantage of the physical flotation method is that it requires high fineness of grinding, and it also discharges a large amount of flotation tailings. More importantly, the dolomite particles finely dispersed in the phosphate rock cannot be extracted to reduce the MgO content in the ore. The degree of satisfaction, as well as the high cost of physical mineral processing investment. According to the flotation results of the Sichuan Maqi Phosphorus Mine and the Hunan Xixi Phosphate Mine (both high-magnesium phosphate deposits), the physical beneficiation can minimize the MgO in the phosphate rock to 1.8%-3.5%. There are still considerable difficulties in the use of ore for acid processing, especially for the processing of feed grade calcium hydrogen phosphate. The MgO contained in the mine will eventually enter the final product in the form of MgHPO4·3H2O. As a result, the calcium content in the product cannot be reached. Industry Standard. The use of the chemical magnesium reduction method, because the basic principle is not to use the different optionalities of the mineral components, but to use the difference in the rate of acid decomposition of the mineral components in the acidic medium and the fact that H+ can diffuse through the fine pores in the ore. The surface of the dolomite particles causes the dolomite in the ore to decompose almost completely under appropriate conditions, while the fluoroapatite in the ore remains therein. Therefore, the removal of MgO in the mine is more thorough, and can be reduced to 0.5%-1.0%. The phosphoric acid is extracted with the phosphate rock, and the quality of the finished phosphoric acid is good. Due to the low content of MgO in the acid, the purity of the feed grade calcium hydrogen phosphate can reach over 95%, and the indicators, especially the calcium content, are relatively easy to reach the industry standard.

The basic principle of chemical magnesium reduction is to wash a high-magnesium phosphate ore containing a large amount of dolomite-type magnesia with a small amount of sulfuric acid, so that the dolomite particles finely dispersed in the phosphate ore particles are dissolved in dilute sulfuric acid, while the phosphate ore is Retained in the solid phase, the main chemical reactions are:

(Ca·Mg)(CO3)2+2H2SO4 CaSO4·2H2O↓+MgSO4+2CO2↑

Since the Mg SO4 formed during the reaction is dissolved in water, it is present in the solution. After appropriate measures are taken to carry out liquid-solid phase separation of the reaction slurry, the dolomite-type magnesium oxide in the solid phase phosphate ore is all or most It is removed, so that the high-magnesium phosphate containing a large amount of magnesium oxide becomes a high-quality phosphate rock suitable for acid processing.

The Phosphate Compound Fertilizer and Phosphate Research Laboratory of Sichuan University (Chengdu Science and Technology University) began systematic research on chemical precipitation pretreatment of magnesium and wet-process phosphoric acid to prepare feed-grade calcium hydrogen phosphate in the early 1980s. After more than ten years of hard work, it has finally achieved success. The chemical pretreatment of magnesium reduction was carried out in Sichuan Mabian Phosphorus Mine. After the mold test and the industrial scale expansion test, the results were all successful. In March 1988 and May 1989, the provincial level was adopted by national experts. Technical appraisal, won the praise. In 1994, the Young Chengdu University-level University Phosphate Compound Fertilizer and Phosphate Research Laboratory provided technology and assumed engineering design. A 25kt/a chemical magnesium reduction device was built in Sichuan Hanyuan Chemical General Plant. This is the first high-magnesium phosphate chemical in China. The magnesium-reducing industrial device passed the expert acceptance in 1995 and evaluated its technology to reach the "international advanced level". The plant has now expanded to a production capacity of 50 kt/a.

4, silicon and acid insoluble content

Phosphate ore usually contains unequal amounts of silicon, mostly in the form of acid insolubles dominated by SiO2. SiO2 does not consume sulfuric acid during the reaction, and some SiO2 can also make the highly toxic gas HF into a less toxic gas of SiF4 gas, reducing toxicity and reducing corrosion. During the reaction, SiO2 is partially eroded by the HF generated by the reaction. The degree of erosion is related to the physical properties of silicon. The more active silicon is easily reacted to produce hydrofluoric acid (HF) to form fluorosilicic acid (H2SiF6). The corrosiveness of metal materials is much lighter than the former. For this reason, the phosphate rock should contain the necessary SiO2. When SiO2/F is less than the stoichiometric amount, soluble silicon should also be added. However, excessive SiO2 is harmful. On the one hand, the colloidal silicic acid in the wet-process phosphoric acid affects the filtration separation of the phosphogypsum; on the other hand, the hardness of the phosphate rock is increased, the mill production capacity is reduced, and the equipment wear is increased.

Fine acid insoluble particles (-320 mesh, ie particles below 44 microns), also known as dispersible mud, will cause great harm to the production of phosphoric acid and phosphate fertilizer. The phosphate rock after enrichment treatment always removes the muddy first, so the muddy effect is not significant. However, when using raw ore without any treatment, the effect of mud is obvious. The fine-grained muddy material in the phosphate rock is not decomposed by sulfuric acid during the reaction process, and precipitates together with the phosphogypsum. It is easy to block the pores of the filter cake filter cloth when the slurry is filtered, greatly reducing the filtration speed and shortening the service period of the filter cloth. It brings serious difficulties to the processing. Even a shale content of only 4% to 5% of phosphate rock can have a significant impact on filtration. For example, when a raw ore with a high dispersive shale content (>10%) in Yunnan is used as a raw material for wet hair phosphoric acid production, the filtration strength of phosphogypsum is only about 200Kg (dry)/m2·h, which is far below the minimum required by the industry ( >500Kg (dry) / m2 · h). It can be seen that the harmfulness of the dispersed muddy rock in the phosphate rock cannot be ignored, and the phosphate rock containing more muddy should be treated with water-eluting mud as much as possible.

5, the content of organic matter and phosphate

Most phosphates, especially sedimentary phosphates, often contain organic matter. High organic content can cause great troubles in operation. Carbonate and organic matter cause bubbles to be reflected in the process, and the organic matter also causes the CO2 gas formed by the reaction to form a stable foam, which is generally called a foam phenomenon. The foam reduces the effective volume of the extraction tank and also causes difficulties in the reaction of the phosphate rock, the transport and filtration of the slurry. In addition, the organic matter is formed into extremely fine carbon particles by carbonization, which easily blocks the filter cloth, reduces the porosity of the filter cake, and lowers the filtration strength. Organic matter also affects the acidity of the product. Therefore, the foam phenomenon is also one of the indicators for evaluating the characteristics of phosphate rock.

The organic matter and carbonate in the phosphate rock are preferably removed by calcination at a calcination temperature of 650-1000 ° C (containing organic matter). The burning of organic matter can partially reduce fuel consumption. Phosphate ore can also improve physical properties after calcination. For wet hair phosphoric acid production, the use of calcination provides the following benefits:

(1) Improve the grade of phosphate rock and reduce transportation costs;

(2) No or less foaming agent may be used;

(3) Improve P2O5 recovery rate and production capacity;

(4) reducing the grinding cost of phosphate rock;

(5) Improve the quality of phosphoric acid and phosphogypsum.

The main disadvantage of the phosphate rock after calcination is that the reactivity is reduced, but this has little effect on the production of wet hair phosphoric acid. In recent years, due to the worldwide energy shortage, methods for calcining phosphate rock have been rarely used. For some carbonate-rich high-concentration phosphates, calcination-digestion can be considered for beneficiation.

6, other components

Fluorine is the main component in the composition of phosphate rock, and it usually exists in a certain proportion with the content of P2O5. Therefore, the fluorine content in phosphate rock is generally not used as an indicator for evaluation. But pay attention to the chlorine content in the phosphate rock. Because chlorine is more serious than fluorine. When the content is slightly higher, the requirements for equipment materials are higher. Therefore, in forests with a chloride content of 0.05% or more, special materials are required when processing by acid method. The general requirement is that when the H2SiF6+HF in phosphoric acid is <2% (in terms of F), the chloride content should not exceed 800 ppm.

The content of manganese, vanadium , zinc and other elements in phosphate rock is generally small, which has little effect on product quality, and is also a trace element required by crops, and has certain fertilizer effects. The rare elements such as uranium in phosphate rock will damage human health through long-term exposure. The necessary protective measures should be taken. When the content reaches 120ppm, it can be recycled during processing.

3. Comprehensive evaluation of phosphate rock quality

Evaluating the quality of a phosphate rock can not only consider the grade of phosphate rock, but also the content and optionality of its harmful impurities (optional, that is, by conventional abdominal selection or other simple beneficiation process) Good quality, lower cost concentrate, reactivity (scale of dissolution of phosphate rock by acid (pure H3PO4)), resistance to retardation (film formation of phosphate rock in mixed acid of sulfuric acid and phosphoric acid) That is, it is resistant to the characteristics of the "passivation film" package, and the foaming property (foaming phenomenon of the phosphate rock in the acid hydrolysis process) is comprehensively evaluated.

The level of phosphate rock grade is mainly reflected in the post-processing economic factors, because some phosphate mines with too low grade, even if technically feasible, the economic indicators are too bad, it is not suitable. The content of harmful impurities in phosphate rock is often the main factor determining whether the post-processing technology is technically feasible. If the technical effect is not feasible in the post-processing process due to the influence of harmful impurities, the economic quality of the phosphate ore grade is not Existed. The level of phosphate rock and the level of harmful impurities are two different concepts, and there is no fixed relationship between them. However, in general, phosphate ore with a high grade contains relatively little impurities.

The selectivity, reactivity, resistance to retardation and foaming of phosphate rock, although it is also a factor in evaluating the quality of phosphate rock, is secondary to the two most important influencing factors of phosphate grade and harmful impurity content. Therefore, generally do not evaluate, only in special circumstances to be considered comprehensively.

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