Method for preparing chromic anhydride from potassium chromate

Abstract

The invention relates to a method for preparing chromic anhydride from potassium chromate. The method comprises the following steps: 1) dissolving and acidizing the potassium chromate to obtain saturated potassium dichromate solution; 2) adding concentrated sulfuric acid in the potassium dichromate solution in the step 1 so that the mole ratio of the dichromate solution to the concentrated sulfuric acid is 1:1-7, stirring, reacting for 0.5-2 hours at the temperature of 20-150 DEG C, precipitating chromic anhydride crystals, filtering to obtain chromic anhydride macro crystals and mother liquor; 3) feeding 40%-60% of mother liquor in the step 2) back to the step 1) for the acidification of the potassium chromate, and carrying out crystallization and reduction on the residual parts to prepare chromic oxide; and 4) rinsing and filtering the chromic anhydride macro crystals in the step 2) with saturated chromic anhydride solution to obtain the chromic anhydride crystals and filtrate, recycling the filtrate to rinse the chromic anhydride macro crystals, and drying the chromic anhydride macro crystals to obtain the chromic anhydride. The invention has the advantages that the method is operated at the temperature lower than the decomposition temperature of the chromic anhydride and under normal pressure, and the obtained product has the advantages of high quality, high yield and low energy consumption.

Description

technical field [0001] The invention relates to the field of inorganic salt production, in particular, the invention relates to a method for preparing chromic anhydride from potassium chromate. Background technique [0002] Chromium salt is one of the main products of inorganic salts, and about 15% of the products in my country's national economy are related to chromium salt. In 2010, my country's chromium salt production capacity reached 400,000 tons, accounting for more than 50% of the world's total production capacity, becoming the world's largest chromium salt producer. Chromic anhydride is one of the main products of chromium salt series, an important inorganic chemical raw material, a raw material for the manufacture of chromium oxide green and zinc chrome yellow; a raw material for the production of high-low temperature conversion catalysts and high-pressure methanol synthesis catalysts; It is used as an oxidizing agent in the industry; it can be used for wood preser...

AI Technical Summary

Problems solved by technology

Electrolysis belongs to the cleaning process, but due to the large power consumption and high cost, it is difficult for manufacturers to accept it, and Na + If the concentration is too high, the electroplating quality of the product is not very good; the current industrial production method of chromic anhydride is the sodium dichromate melting method, and the reaction is carried out in two stages: the first stage (about 170°C) releases free water and sodium dichromate The water of crystallization in the molecule; the second stage (170-197°C) releases chemical reaction water, and the temperature at the end of the reaction is controlled at 200-205°C. At this time, the solid phase in the reaction material is completely melted (CrO 3 Melting point is 197°C, NaHSO 4 Melting point is 186 ℃), and the temperature slightly higher than the melting temperature of chromic anhydride is beneficial to the stratification of reaction materials; if the temperature is lower than 200 ℃, the two melts are difficult to stratify because of the increase of melt viscosity; when the temperature lower than NaHSO 4 When the melting point is 186°C, the reaction equation is: Na 2 Cr 2 o 7 (Crystal)+2H 2 SO 4 (liquid)→2CrO 3 (Crystal)+2NaHSO 4 (liquid)+H 2 O (steam); when the temperature is higher than CrO 3 When the melting point is 197°C, the reaction equation is: Na 2 Cr 2 o 7 (Crystal)+2H 2 SO 4 (liquid)→2CrO 3 (liquid)+2NaHSO 4 (liquid)+H 2 O (steam)

for K 2 Cr 2 o 7 and concentrated H 2 SO 4 As far as the reaction is concerned, if the melting method is used, because the generated KHSO 4 The melting point is 219°C. Below this temperature, it cannot be operated; above this temperature, it is difficult to separate the two products after they are completely melted, which brings great difficulty to the separation and is basically unfeasible in industry.

Therefore, although the melting method is currently the most important production method in all countries in the world including my country, the temperature of the material is higher than the melting point of chromic anhydride in the whole preparation process (reaction, evaporation and melting, clarification and separation, and sheet production). The chromic anhydride in the state is unstable and easily decomposes, resulting in an increase in the water-insoluble matter of the product, high turbidity and a decrease in quality, reducing the product yield

[0004] The crystallization method is also a clean process and is recognized as the most advanced method in the world, but only one company in the UK has this technology, and the technical solution has not yet been made public.

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