Study on preparation technology of high purity cerium hydroxide

China's rare earth reserves rank first in the world, of which cerium accounts for about 50% of the total rare earth reserves. Cerium hydroxide is the largest tetravalent cerium product. Cerium hydroxide is non-toxic. High purity cerium hydroxide is used as an additive for color TV fluorescent screen, which can change the toughness, strength and other optical properties of fluorescent glass, and there is a great demand in the market; Cerium hydroxide used as catalyst for automobile exhaust purification can partially replace precious metals because of its unique OSC effect; Cerium hydroxide can also be used as a decolorizing clarifier for glass and to prepare ammonium ceric nitrate, ceric sulfate, ammonium ceric sulfate reagents, etc. With the further development of rare earth industry, higher requirements are put forward for the purity and various properties of cerium hydroxide products.

  there are many studies on the preparation of cerium hydroxide. Cerium hydroxide with purity greater than 99.9% can be prepared from mixed rare earth solution. Cerium hydroxide meeting the market demand can be prepared by ammonia hydrogen peroxide oxidation precipitation method, and cerium hydroxide ultrafine powder can also be prepared. Some scholars have also proposed a green chemical method for preparing cerium hydroxide. These studies mainly focus on the synthesis process, but pay less attention to the comparative analysis, structural composition, performance characterization and product cleaning and purification. In this paper, the effects of various conditions of ammonia and hydrogen peroxide oxidation precipitation method on the cleanliness of cerium hydroxide are studied. The two processes of cerium double salt alkali conversion and cerium carbonate alkali conversion are compared, and the structural composition of the product is determined by thermal decomposition reaction.

Cerium hydroxide

1   test part

1.1   raw materials and reagents

Cerium carbonate (w (CE2 (CO3) 3) ¡Ý 99.99%), nitric acid, ammonia and hydrogen peroxide are all analytical pure.

1.2   analysis method

   the rare earth concentration in the solution is determined by EDTA complexometric titration, the cerium concentration is determined by ferrous sulfate redox titration, the heavy metal impurities such as iron, cobalt, nickel, chromium, lead, zinc and copper are determined by plasma mass spectrometry, and the rare earth impurities are determined by plasma mass spectrometry. The morphology of cerium hydroxide was taken by S-3400 scanning electron microscope, and the particle size was determined by coulterls230 laser particle size analyzer, The TG-DTA experiment was carried out with Shimadzu dt-2a differential thermal analyzer and its TB-2 thermal balance additional device (air, heating rate of 5 ¡æ / min, Al2O3). The XRD pattern was obtained by Philips pw-1700 X-ray automatic powder diffractometer in the Netherlands.

Cerium oxide

1.3   sample preparation

   dissolve cerium carbonate (99.99%) with nitric acid and prepare cerium nitrate solution with a certain concentration, which is filtered and ready for use. Under the stirring condition, hydrogen peroxide and ammonia are added to the cerium nitrate material solution for oxidation precipitation. After the reaction, it is heated, filtered, washed and dried to obtain a clean cerium hydroxide product.

2   test results and discussion

2.1   effect of reaction conditions on oxidation rate, yield and mass fraction of heavy metal impurities of cerium hydroxide during oxidation precipitation

  cerium carbonate is mixed with nitric acid and dissolved into cerium nitrate solution. Adding hydrogen peroxide and ammonia to cerium nitrate solution, trivalent cerium ion is oxidized to reddish brown cerium peroxide precipitation, and nitric acid is generated at the same time. Ammonia can regulate the acidity of the system and ensure the smooth progress of redox reaction.

Ammonium nitrate is generated in the mother liquor, and cerium peroxide is transformed into cerium hydroxide after heating. The chemical reaction equation is as follows:

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   during the oxidation precipitation reaction, the change of reaction conditions will have a great impact on the oxidation rate and yield of cerium hydroxide [14]. The single factor condition test was carried out by selecting the concentration of the feed solution, the acidity of the system, the amount of hydrogen peroxide, the oxidation temperature, the oxidation time, the washing method and so on. Fig. 1 ~ 6 are the relationship curves between different reaction conditions and cerium hydroxide oxidation rate, yield and mass fraction of heavy metal impurities in the oxidation precipitation process.

The pH of the system is 7, the amount of hydrogen peroxide is 24ml, the oxidation temperature is normal temperature, the oxidation time is 30min, and the washing method is pure water washing once + absolute ethanol washing once.

Fig. 1 Relationship curve between mass concentration of cerium nitrate and oxidation rate, yield and mass fraction of heavy metal impurities of cerium hydroxide

The mass concentration of cerium nitrate is 80g / L, the amount of hydrogen peroxide is 24ml, the oxidation time is 30min at room temperature, and the washing method is pure water washing once + absolute ethanol washing once.

Fig. 2   relationship curve between system pH and cerium hydroxide oxidation rate, yield and mass fraction of heavy metal impurities

The mass concentration of cerium nitrate is 80g / L, the pH of the system is 7.0, the oxidation temperature is normal temperature, the oxidation time is 30min, and the washing method is pure water washing once + absolute ethanol washing once.

Fig. 3   relationship curve between hydrogen peroxide addition and cerium hydroxide oxidation rate, yield and mass fraction of heavy metal impurities

The mass concentration of cerium nitrate is 80g / L, the pH of the system is 7.0, the amount of hydrogen peroxide is 24ml, the oxidation time is 30min, and the washing method is pure water washing once + absolute ethanol washing once.

Fig. 4 Relationship curve between oxidation temperature and cerium hydroxide oxidation rate, yield and mass fraction of heavy metal impurities

The mass concentration of cerium nitrate is 80g / L, the pH of the system is 7.0, the amount of hydrogen peroxide is 24ml, the oxidation temperature is normal temperature, and the washing method is pure water washing once + absolute ethanol washing once.

Fig. 5   relationship curve between oxidation time and cerium hydroxide oxidation rate, yield and mass fraction of heavy metal impurities

It can be seen from Figure 1 that with the increase of cerium nitrate concentration in the feed solution, the oxidation rate and yield of cerium hydroxide have a downward trend, but the overall change is small. The oxidation rate remains above 98% and the yield is stable between 96% ~ 98%; The mass fraction of heavy metal impurities is stable at (22 ~ 25) ¡Á 10-6. The gas released during the oxidation precipitation reaction and the increase of the mass concentration of the feed liquid can improve the utilization rate of the equipment, but it is not conducive to the reaction. Through comprehensive consideration, it is determined that the mass concentration of cerium nitrate in the feed solution is 80g / L.

   it can be seen from Figure 2 that the oxidation rate of cerium hydroxide decreases with the increase of feed solution pH, indicating that it is not conducive to the oxidation of Ce3 + under strong alkaline conditions, and excessive ammonia will precipitate Ce3 +; The yield increased first and then decreased, indicating that the oxidation reaction was incomplete and the concentration of Ce3 + in the solution was high; The mass fraction of heavy metal impurities has an upward trend, indicating that under alkaline conditions, heavy metal impurities will enter cerium hydroxide in the form of hydroxide. When the pH of the system increases, the ammonia consumption increases, and excessive ammonia will increase the volatilization loss and affect the operating environment. Therefore, it is appropriate to determine the pH of the system at 7.0.

   it can be seen from Figure 3: with the increase of hydrogen peroxide dosage, cerium hydroxide oxidation increases first and then tends to be stable; The yield decreased slightly because the acidity of the system increased after adding excess hydrogen peroxide. Under acidic conditions, hydrogen peroxide reduced cerium peroxide to Ce3 +; The mass fraction of heavy metal impurities increased slightly, indicating that heavy metal impurities are easy to be entrained into cerium hydroxide under acidic conditions. After comprehensive consideration, 24ml hydrogen peroxide was determined as the best amount.

   it can be seen from Figure 4: with the increase of oxidation temperature, the oxidation rate of cerium hydroxide decreases slightly. Too high temperature will decompose hydrogen peroxide and cannot play the role of oxidation; The oxidation precipitation reaction is an exothermic reaction. After the reaction, the system temperature increases by about 5 ¡æ, which is not conducive to the reaction; With the increase of temperature, the yield of cerium hydroxide decreased, and a large amount of Ce3 + remained in the mother liquor; yield

The mass fraction of heavy metals in the product changes little and is less affected by temperature. After comprehensive consideration, it is determined that the reaction can be carried out at room temperature.

   it can be seen from Figure 5 that the oxidation rate of cerium hydroxide changes little with the extension of oxidation time, and the oxidation precipitation reaction can be completed in a short time; The yield remained between 96% ~ 98%, indicating that once the oxidation precipitation reaction was completed, there would be no dissolution change due to the extension of time; The mass fraction of heavy metal impurities increases gradually, indicating that heavy metal impurities will gradually enter cerium hydroxide with the progress of the reaction. The oxidation time is prolonged, the production efficiency is reduced and the cost is increased. Therefore, the reaction time should be determined as 30min.