Preparation And Structural Regulation Of Coal Gangue-based Zeolites And Their Pollutant Treatment Performance

Coal is one of the most important energy resources in China,the mining and utilization of coal has brought huge economic benefits to the society,but at the same time,it has also produced a series of ecological and environmental problems.Among them,the impact of coal gangue on the ecological environment is particularly prominent.Coal gangue is the largest emission of solid waste in China,and its large-scale accumulation occupies a large amount of land and also causes serious pollution to the environment,so how to deal with the disposal of coal gangue is a major issue of great concern to the current government at all levels,academia and industry.Hydrothermally modified coal gangue preparation of zeolite-based environmental protection materials can achieve the effective utilization of coal gangue,zeolite-based materials and the preparation of low-cost environmental pollution management,is a very promising coal gangue high-value utilization of one of the ways.However,there are still two key problems to be solved:one,the gangue has a complex structure,high impurity content,low reactivity and other problems,making the preparation of gangue-based zeolite is not high purity,unstable performance,poor application.Therefore,how to solve the above problems,the development of coal gangue impurity removal and activation technology and clarify the mechanism is one of the key issues to be solved.Second,the preparation process of gangue-based zeolite is mostly tedious and complicated,which is not conducive to the large-scale industrial use of gangue-based zeolite.Therefore,the development of efficient preparation technology of gangue-based zeolite and the clarification of its related mechanism is another key problem that needs to be solved.In response to the two major problems,this research has been carried out systematically and thoroughly,and the main contents and results are as follows:(1)Using coal gangue from Yulin,Shaanxi Province,acid/heat treatment was carried out to reduce its impurity content and enhance its reactivity,and the effects of acid treatment type,concentration and sequence of acid/heat treatment were investigated to clarify the mechanism of different pretreatment means on the composition structure of coal gangue and coal gangue-based zeolite.The study showed that acid treatment can effectively reduce the content of CaO,Fe2O3 and impurities such as Cr and Mn in gangue,and heat treatment can effectively reduce the content of calcite and transform kaolinite into more active metakaolinite;after treatment with 4 mol/L hydrochloric acid and then heat treatment at 850℃,the content of CaO and Fe2O3 in gangue was reduced by 5.79 wt%and 2.31 wt%,respectively.Cr and Mn element content were reduced by 0.0617 wt%and 0.0657 wt%,respectively;heat-treated and acid/heat-treated gangue could be converted into sodalite under 6 mol/L NaOH hydrothermal modification conditions,in which the impurity content of acid/heat-treated gangue hydrothermal products was significantly lower than that of heat-treated gangue hydrothermal products.(2)Several gangue-based zeolites with low silica-alumina ratio and high cation exchange performance were successfully prepared by one-step hydrothermal alkali treatment modification of the pretreated gangue.The mechanism of hydrothermal temperature and sodium-silica ratio on the composition and structure of hydrothermal products was studied,and the removal effect of gangue-based zeolites on cadmium and methylene blue in water and the removal mechanism were clarified.The study shows that the transformation of gangue to zeolite can be achieved by regulating the key influencing factors such as sodium-silica ratio and hydrothermal temperature.When the hydrothermal temperature is 150℃and the sodium-silica ratio is 15,the hydrothermal products are mainly sodalite;when the hydrothermal temperature is 180℃ and the sodium-silica ratio is 15,the hydrothermal products are mainly faujasite.The adsorption capacities of the prepared sodalite and faujasite for cadmium in water were 40.9 and 36.4 mg/g,respectively,which were 10 and 9 times higher than those of coal gangue;the adsorption equilibrium time for methylene blue in water was 10 min,and the adsorption rate was significantly higher than that of coal gangue.(3)Natural plagioclase zeolite,β zeolite,Y-type zeolite,4A zeolite,mercerized zeolite and ZSM-5 zeolite crystal species were added to the hydrothermal alkali treatment system of coal gangue,and the effects of zeolite crystal species on the composition structure and performance of hydrothermal products and their mechanisms were investigated,and the removal effects of hydrothermal products on cadmium and chromium ions in water and their mechanisms were analyzed.The study showed that the zeolite crystal species had a great influence on the composition structure and performance of the hydrothermal products.Under 6 mol/L hydrothermal conditions,the hydrothermal products after adding zeolite crystal species were all sodalite,while under 2.5 mol/L hydrothermal conditions,the hydrothermal products after adding zeolite crystal species were square zeolite,sodalite and P-type zeolite.When 20%natural plagioclase zeolite crystal seeds were added,the adsorption capacities of the hydrothermal products obtained after 6 mol/L hydrothermal treatment could reach 47.47 and 39.21 mg/g for cadmium and chromium ions,which were 1.5 and 1.4 times higher than those of gangue-based zeolites without the addition of natural zeolite crystal seeds,respectively.