Removal of Magnesium Oxide from High Magnesium Phosphate Ores from Phosphate Chemical Acid Wastewater

Phosphate rock is an important mineral resource, mainly used in the production of phosphoric acid, phosphate fertilizer and phosphate products. The presence of impurities such as magnesium, aluminum, iron and other impurities in phosphate rock, especially magnesium, will bring a series of extremely adverse effects on the wet processing of phosphate rock. In the production of wet phosphoric acid, almost all of the magnesium enters into the phosphoric acid, which not only increases the consumption of sulfuric acid, but also causes fine crystals of calcium sulfate, which reduces the filtration strength of phosphogypsum, and reduces the washing efficiency and phosphorus yield. For a production unit of the same scale, its actual production capacity decreases with the increase of magnesium content in the phosphate rock. Not only that, after the magnesium in the phosphate rock enters the phosphoric acid liquid phase, it also adversely affects the subsequent processing of the phosphoric acid. When the phosphoric acid is concentrated, the viscosity of the acid is increased and the precipitate is easily blocked. Problems such as grain difficulties and insufficient nitrogen nutrients in products, and insufficient calcium nutrients in products when producing calcium phosphate salts.

For the removal of impurities including magnesium in phosphate rock, physical methods such as scrubbing, forward and reverse flotation and heavy medium selection are mainly used at home and abroad to separate phosphate rock and impurities, so as to achieve the purpose of enriching phosphate rock and removing impurities . However, it is difficult to reduce the MgO content in the phosphate rock to less than 1% by only using the method of physical beneficiation, and the phosphorus loss is relatively large. Therefore, other treatment methods need to be studied, and for the phosphate rock with high magnesium content, the research is effective. The method of reducing magnesium oxide content is more urgent. Chemical demagnesization mainly uses the principle of “weak acid demagnesization. Magnesium in phosphate rock is basically in the form of dolomite (CaMg(CO3)2), and the reactivity of CaMg(CO3)2 in weak acid medium such as dilute sulfuric acid is used. It is far greater than the characteristics of calcium fluorophosphate, so that H+ penetrates into the surface of dolomite particles through the fine pores of phosphate rock particles to participate in the reaction, so as to achieve the purpose of decomposing CaMg(CO3)2 and removing Mg, thereby reducing the magnesium content in the phosphate rock.

In the production process of phosphorus chemical industry, a large amount of acidic wastewater will be produced. The wastewater is generally strong in acidity and high in phosphorus and fluorine content. It needs to be treated before it can be recycled as process water or discharged to the standard. Studying the utilization of this acid wastewater can reduce the treatment load of enterprise sewage, and at the same time achieve the effect of energy saving and emission reduction, and the environmental protection benefits are remarkable. This project mainly uses the acid wastewater of phosphorus chemical industry to carry out research on the removal of magnesium oxide from high-magnesium phosphate rock through the principle of chemical removal of magnesium.


1.1 Experimental materials

The high-magnesium phosphate rock samples used in the experiment were taken from Fuquan, Guizhou, and the chemical composition analysis is shown in Table 1.


The acid wastewater of phosphorus chemical industry used was taken from Machangping Industrial Park, Fuquan, Guizhou, and the main components are shown in Table 2.


1.2 Experimental method

According to a certain liquid-solid ratio, high-magnesium phosphate rock is added to the acidic wastewater at a certain temperature, and the reaction is continuously stirred for a certain period of time. After the reaction, the solid-liquid separation was carried out by suction filtration, the content of phosphorus and magnesium in the solid phase was analyzed, and the change of phosphorus and magnesium in the phosphate rock was calculated.

1.3 Analysis method

The mass fraction of phosphorus pentoxide in the phosphate rock was determined by the quinoline phosphomolybdate mass method, and the mass fraction of magnesium oxide in the phosphate rock was determined by the EDTA volumetric method.

Results and discussion

2.1 Optimization of reaction temperature

On the basis of the accumulation of exploration experiments, firstly, under the process conditions of liquid-solid ratio of 4:1 and reaction time of 1.5h, the optimization process experiments of reaction temperature of 25℃, 30℃, 35℃, 40℃, and 50℃ were carried out. The experimental results are as follows: shown in Table 3.


It can be seen from Table 3 that the temperature has a certain influence on the demagnesization rate. The demagnesization rate increases first and then decreases slightly with the temperature. The magnesium oxide in the post-middle phosphate rock is only 0.94%. From the viewpoints of the demagnesization effect, energy saving, and ease of industrialization, it is considered appropriate to set the chemical demagnesization reaction temperature to about 30°C.

2.2 Optimization of reaction time

Under the process conditions, the liquid-solid ratio is 4:1, and the reaction time is optimized for 0.5h, 1h, 1.5h, and 2.5h under the reaction temperature of 30 °C. The experimental results are shown in Table 4.


It can be seen from Table 4 that the longer the reaction time, the higher the magnesium removal rate. When the reaction time reaches 1h, the magnesium removal rate tends to be stable and grows slowly.

When the reaction time exceeded 1.5h, a small amount of fluorapatite began to react with the acidic wastewater, so the phosphorus recovery rate began to decline.

Through experiments, the preferred experimental reaction time is 1~1.5h, and appropriate adjustments can be made according to the actual production. For example, when the magnesium oxide content in the high-magnesium phosphate rock is high or the requirement for the removal rate of magnesium is high, the reaction time can be selected to be 1.5 h, on the contrary, when the magnesium oxide content in the phosphate rock is relatively low or the requirement for the removal rate of magnesium is not high, the reaction time can be selected as 1h.

2.3 Optimization experiment of liquid-solid ratio

Under the conditions of reaction temperature of 30°C and reaction time of 1.5h, the liquid-solid ratio optimization experiments of acidic wastewater and high-magnesium phosphate rock with liquid-solid ratios of 2:1, 3:1, 4:1, and 5:1 were carried out. The experimental results are shown in Table 5.


It can be seen from Table 5 that the liquid-solid ratio has a great influence on the removal of magnesium from high magnesium phosphate rock. The magnesium oxide content in the phosphate rock is less than 1%.

Compared with phosphate rock, the larger the amount of acidic wastewater added, the more acid that can react with the phosphate rock, and the concentration of liquid-phase magnesium is reduced by dilution, which is also conducive to the demagnesization reaction, so the liquid-solid ratio It is the biggest factor affecting the effect of demagnesia.

From the perspective of industrialization, the liquid-solid ratio is an important factor affecting the size and production capacity of an industrialized device. The larger the liquid-solid ratio, the larger the equipment investment scale. At the same time, after the chemical demagnesization reaction, the liquid-phase magnesium-containing liquid of solid-liquid separation needs to be treated. The ratio of liquid to solid ratio. Therefore, on the basis of satisfying the demagnesization effect of high-magnesium phosphate rock, it is necessary to limit the input amount of acidic wastewater, and the preferred liquid-solid ratio in the experiment is 4:1.


In this study, acid wastewater from phosphorous chemical industry is used to chemically remove magnesium phosphate rock. The preferred process for removing magnesium is as follows: the reaction temperature is 30 °C, the reaction time is 1-1.5 h, the reaction liquid-solid ratio is 4:1, and the process effect is that the phosphate rock is oxidized Magnesium was reduced from 2.92% to 0.94%, the magnesium removal rate was about 70%, and the phosphorus recovery rate was about 100%. Experiments have shown that this technology can effectively reduce the content of magnesium oxide in high-magnesium phosphate rock, improve the economic value of phosphate rock after demagnesium treatment, and reduce the adverse effect of magnesium oxide in phosphate rock on acid and fertilizer production from the source. The treatment and utilization of phosphate rock provides an effective way.

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