| Chromium oxide is one of the main products of chromium chemical industry. Metallurgical grade chromium oxide is the raw material for preparing high purity metal chromium. Chromium oxide green pigment (Cr2O3) has good thermal stability, high temperature resistance and weather resistance, it is widely used in ceramics, paint, building materials and other industries. The research of production engineering of chromium oxide is an important part of liquid phase oxidation process of potassium salt and the cleaning process and the integrated technology of chromate. This paper is based on the preparation process of chromium oxide through the hydrogen reduction of potassium chromate (K2CrO4). The influence law of color properties, control method and stability preparation method of low cost were systematic studied, and the industrial expansion experiment was also accomplished. The stability of the pigment grade Cr2O3 product can be controlled.The following achievements and innovative progresses were exhibited:(1) The influence of chemical composition of precursor, grain size, particle size distribution and ion doping on the color performance of chromium oxide green were systematic studied. It was found that: ① Amorphous y-CrOOH produced more homogeneous Cr2O3 with a yellowish-green colour, which can be compared with commercial Cr2O3 products at home and abroad (L*=50.12, a*=-18.48, b*=20.20). α-CrOOH produced more Cr2O3 with a dark-green colour (L*=49.02, a*=-12.07, b*= 11.97); ② By adjusting the calcination conditions and grain size, chromium oxide productd with yellow-green and blue-green color performance were prepared. The color of Cr2O3 was bright yellow-green when the grain size was 350nm, while the color was dark blue-green when the grain size was 500nm. The color properties of the-a*(green) and b*(yellow) can be improved by changing the particle size distribution from two peaks to a single peak. The color properties of the-a*(green) and b*(yellow) can be also improved by addition 1～4wt% boric acid in the precursor; ③ By changing the proportions of α-CrOOH and γ-CrOOH in the precursor, the grain size and particle size distribution, the resulting pigments can exhibit a variety of controllable colours, including:bright yellow green, bule green, dark green and pure green.(2) The hydrogen reduction reaction mechanism was first analyzed and the process was optimezed. By using in situ XRD and in situ FTIR, the reducing process of K2CrO4 was also studied. The results show that the reduction of K2CrO4 was a very complicated process, including three solid regions:K3CrO4+ amorphous Cr3++KOH, amorphous CrOOH+KOH, KCrO2+K0.7CrO2. By adjusting the reduction temperature and time, the different composition of the reduction products were prepared.(3) The effect of hydrolysis reaction mechanism of three reduction products and the composition of hydrolysis products on the color performance of chromium oxide products were studied. It was found that K3CrO4 quickly reacted with water to forming two-thirds K2CrO4 (CrⅥ) and one-third green gel material (CrⅢ). The K. content of green gel material was very low. The γ-CrOOH did not react with water, and the K and Cr in the solid products were completely dissociated. The hydrolysis reaction of KCrO2 was very slowly, and α-CrOOH was prepared after hydrolysis. The K and Cr in hydrolysis product were not completely dissociated, and the K content was high.(4) The effects of the type of precursor, thermal decomposition temperature and other factors on the NIR reflectance of Cr2O3 were studied. The results demonstrated that the produced Cr2O3 had a higher NIR reflectance of 84% and good yellowish-green color compared to the dark-green standard Cr2O3. Meanwhile, TiO2 was doped to improve the NIR reflectance (as high as 91.25%) and a serious of Cr2-xTixO3+δ (x ranges from 0 to 0.02) having a corundum structure displaying color of bright yellowish-green was synthesized by the solid state route.(5) The industrial enlarged experiment of the production of pigment grade chromium oxide was studied. Through study the parameters of steel reduction furnace, the production process was optimized, the hydrogen reduction reaction process and hydrolyzed product components were controled effectively. The reduction rate was 95% and the yield was more than 85%, and the production cost was reduced. Hydrocyclone was used to separate the reduction products, the ratio of coarse material (γ-CrOOH) was effectively improved, and the size of precursor was controled. The color performance of the chromium oxide products can be comparabled to foreign pigment-grade chromium oxide product, and the process was stable and feasible. |
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