Objectives  The technology of producing Se fertilizer was studied using Se-rich coal gangue from solid waste ore.

Methods The tested coal gangue was from Enshi, where was famous for rich of Se in the environment, and grounded to pass 0.038 mm for the experiment. Preliminary experiments were conducted with 6 levels of each single factor for the activation of Se, and three levels of each were accordingly selected for the following response surface optimization, which would give the optimal activation parameters eventually. The IR spectroscopy and TG thermogravimetry methods were employed to observe and analyze the activating and thermal effects of the bonds of Se with other metals before and after the activation in the coal gangue. The activated coal gangue was then blended with farm manure in ratio of 1∶1, 2∶1, 3∶1 and 4∶1, and fermented for one month to make activated coal gangue selenium organic fertilizers (organic Se fertilizer). A garlic field trial was conducted using the four organic Se fertilizers as treatments, and pure activated coal gangue Se and farm manure as controls. The total and organic Se contents in the garlic bulbs were measured at harvest.

Results The single factor levels obtained from the single factor experiments were 10%, 20% and 30% for Na₂CO₃, 2.5 h, 3.5 h and 4.5 h for the activation time, 75℃, 85℃ and 95℃ for the activation temperature, and 1∶5, 1∶10 and 1∶15 for solid-liquid ratio (g/mL). The optimized activation condition by the response surface optimization were Na₂CO₃ 22%, the activation time 3.9 h, the solid-liquid ratio 1∶9 g/mL and the activation temperature 85℃. Under the condition, the pH of the activated coal gangue was 7.6, and the maximum activation rate of Se was about 81.24%, and the active Se content was 170.82 mg/kg. Through the IR spectroscopy image, it could be seen that the bonds between Se and other symbiotic metal and nonmetal elements were broken, and the original crystalline network were broken too, indicating the Se in the coal gangue was unleashed. The derivative thermogravimetry curve had 2 peaks before activation within 100℃, while only 1 peak left near 100℃ after activation, the weightlessness situation was weaker than the non-activated as well, indicating the improved heat stability of the coal gangue. According to the Se content in the activated coal gangue, the Se contents in the prepared 4 Se organic fertilizers were roughly 85.41, 113.88, 128.05 and 136.66 mg/kg in turn. The total Se contents of the garlic bulbs applied with the coal gangue fertilizer and the four Se organic fertilizers were 1.033, 1.306, 1.480, 1.382, and 1.355 μg/g, respectively, which were all significantly higher than that in the none-selenium fertilizer control (P < 0.05). The organic Se rate of the garlic bulbs in the five fertilizer treatments reached 99%, which was higher than the 60% of the control. The four Se organic fertilizers produced significantly higher Se contents in garlic bulbs than the activated coal gangue did, with the highest Se content by the 2∶1 ratio mixed Se organic fertilizer.

Conclusions The optimum process conditions of coal gangue using Na₂CO₃ as activator are: the Na₂CO₃ 22%, activation time for 3.9 h, the solid-liquid ratio of 1∶9 g/mL at 85℃. Under the condition, the activation ratio of Se can be as high as 81.24%, and with good thermal stability. The preparation of Se organic fertilizer made by co-fermentation of farm manure and activated coal gangue can greatly increase the efficiency of the activated Se, and the optimum effect is under the mix ratio of 1∶2.