The Research Of Acidification And Removal Of Aluminium In Chromate Alkaline Solution In The Chromic Salt Production

Chromate is widely applied inorganic chemical material.The chromic salt production in China accounts for～35% of the total yield in the world. calcium-free oxidative roasting process is the technological tendency of the chromate salts production for environmental protection. And then, the controlling of acidification operation becomes more difficult, leading to the high acidification rate of chromate and the large amount of mirabilite. Meanwhile, there was a large amount of aluminium hydrate with high water and chromate content after calcium-free oxidative roasting process adopted. Based on the theory of thermodynamics and nucleation, agglomeration and crystal growth of crystallography, the acidification process of sodium chromate solution and removal of aluminium were studied. Then, some results were summarized as follows.The conversion rate of chromate solution was calculated theoretically based on the principle of ionic balance and electro-neutrality. The effect of the the concentration of Na2CrO4 and Na2SO4 in the solution on the conversion rate and the ion structure change in the solution during the acidification were studied. The results show that the conversion rate increases in the form of "S" curve when the pH value of the chromate solution decreases, and that increasing the concentration of Na2CrO4 in the solution or decreasing the temperature of the solution favors the conversion rate of chromate. In addition, the conversion rate also increases along with the increasing concentrations of Na2SO4.The paper also investigated effects of supersaturation ratio, seed coefficient, pH value, temperature and seed circulation on removal of aluminium in alkaline solution and industrial alkaline solution. The results show that the aluminium hydrate with low content of water and chromate, and slurry with good settlement properties occurs by controlling the supersaturation C. Meanwhile, the water and chromate content of aluminium hydrate decrease and the settlement properties become better by adding seed and increasing pH of neutral chromate solution and temperature.The results in pilot seals production show that water content of aluminium hydrate varies from 35% to 45%, chromate content is less than 1%, and that the slurry performs good settlement properties under the suitable conditions:pH 6.5～10.5, temperature 50～90℃, time 1～5h.The settlement mechanism was discussed based on the analysis Zeta potential, contact angle, particle-size and SEM. The results show that settlement properties tendency relates to solvent tendency order. The Zeta potential of aluminium hydrate decreases when chromate content decreases. The coarse particle-size and regular morphology of aluminium hydrate favors settlement properties of slurry.