Negative temperature effect of complex reaction kinetics system of Fe and Al mineral impurities in liquid–solid process

•Kinetic digestion experiments of Fe and Al mineral impurities of phosphate rock.•Novel negative temperature effect in the kinetic system below a critical point.•The mechanism cased by coupling of chemical reaction, phase change and diffusion.•The theoretical model fitting experimental data satisfactorily.•Percolation transition under threshold in fractal structure. Depletion of phosphate ore is one of the important resource crises in the world. The exploration of the transfer regularity of mineral impurities to liquid in the digestion process of phosphate rock is of great significance to the production of agricultural fertilizer. In this paper, the reaction kinetics experiments of iron and aluminum compounds in phosphate ore particles were carried out in the range of temperature 50–70℃ and phosphoric acid concentration 15–25%. Under the combined temperature and concentration conditions, a large number of experiments were carried out to determine the conversion fraction of impurities, and the relationship between conversion fraction and time was obtained. A novel phenomenon of liquid-solid two-phase reaction kinetics was discovered. That is the negative temperature effect. When acid concentration is higher than the critical point, the reaction conversion fraction increases with increasing temperature, but it decreases with increasing temperature below the critical value. It has been found that this phenomenon is caused by the mutual coupling of the phase transition, mass transfer and chemical reaction in the micro-pores structure, as well as specific solubility characteristics of iron-aluminum phosphate. According to the mechanism analysis, the kinetic model was established, which is in good agreement with the experimental data. It can be used to predict the reaction behavior of iron and aluminum mineral impurities of phosphate rock in phosphoric acid. And the kinetics parameters such as Dpd, γ, Dcop were thus obtained.