Enhanced Metal Leaching For Preparation Of Polymeric Aluminum Ferric Chloride Sulfate(PAFCS) From Circulating Fluidized Bed Fly Ash

With the rapid development of circulating fluidized bed（CFB）combustion technology in China,the amount of circulating fluidized bed fly ash generated is increasing year by year.It has important social,economic and ecological benefits to realize the resource utilization of CFB fly ash.Compared with pulverized coal furnace fly ash,CFB fly ash contains a large amount of iron and calcium,which is not suitable to the utilization of building materials.However,the aluminum,iron and calcium in CFB fly ash are important components for the synthesis of flocculant,and using them to prepare inorganic polymer flocculants can not only relize solid waste resource utilization but also wastewater treatment.At present,the preparation of inorganic flocculants using fly ash has problems such as low aluminum utilization rate and poor flocculant performance.Therefore,this paper proposes to achieve efficient dissolution of aluminum and iron elements in CFB fly ash from Pingshuo coal gangue power plant by hydrochloric acid atmospheric leaching-sulfuric acid pressurized enhanced leaching method,and then prepare polymeric aluminum ferric chloride sulfate（PAFCS）by hydrolysis,polymerization and aging processes.The mechanisms of the influence of the composition in leachate on the structure and performance of PAFCS,and the main results are as follows:On the basis of achieving efficient dissolution of aluminum and iron using hydrochloric acid atmospheric leaching-sulfuric acid pressurization enhanced leaching method,PAFCS was prepared by hydrolysis-polymerization-aging process.The results of hydrochloric acid atmospheric leaching-sulfuric acid pressurized enhanced leaching showed that the leaching rates of Al₂O₃ and Fe₂O₃ were 65.1%and 96.1%using 6 mol/L hydrochloric acid solution at 100°C for 120 min,and then leaching at 160°C for 120 min in 40%sulfuric acid,the total leaching rates of Al₂O₃ and Fe₂O₃ reached 96.75%and 97.9%.After the acid leach solution of hydrochloric acid is neutralized to p H 3.0-3.5 with aluminum hydroxide sol at 60-70°C,the acid leach solution of sulfuric acid is added and held for 4 h.And the reddish-brown PAFCS can be prepared by aging the above mixture for 1 d.The analysis results of XRD and FTIR show that Al,Fe and SO₄^2-are not simply present in the system as ions,but are involved in the polymerization reaction to form new polymers.The influence laws of different Al/SO₄^2-,Al/Fe and Al/Ca molar ratios on the structure and flocculation performance of fly ash-based PAFCS flocculants were systematically investigated.The effect of different Al/SO₄^2-and Al/Fe molar ratios on the structure and performance of PAFCS flocculants was significant,and the appropriate Al/SO₄^2-and Al/Fe molar ratios facilitate the polymerisation reaction of the different reaction units in the system and result in more stable copolymers.When Al/SO₄^2-and Al/Fe molar ratios are 10:1 and Al/Ca molar ratio is 20:1,the content of Al-Fe monomer form is the lowest and the content of medium polymerization state Al-Fe copolymer is the highest,and the performance of the prepared flocculant is better;excessive addition of SO₄^2-and Fe³⁺will make the system unstable when Al-Fe ions are hydrolyzed and polymerized,and it is easy to generate sols or precipitates.The Al/Ca molar ratio has a small effect on the structural performance of PAFCS.The appropriate Ca²⁺addition is beneficial to improve the electro-neutralisation of the flocculant,but when the Ca²⁺content is too high it is easy to combine with SO₄^2-or OH^-in the system to produce calcium sulphate precipitation or calcium hydroxide,which reduces the amount of aluminium and iron that can combine with SO₄^2-or OH^-in the system,which is not conducive to the improvement of flocculation performance.The PAFCS flocculant prepared under optimum conditions was applied to the coking wastewater treatment process and compared with the flocculation performance of commercially conventional flocculants（PAC,PAFC）.Compared to PAC and PAFC,the fly ash based PAFCS flocculant is more effective in treating the biochemical treatment effluent of coking wastewater.The removal rates for turbidity,UV₂₅₄,COD and DOC were 93.2%,47.7%,44.3%and 52.1%respectively,and fulvic acid-like and humic acid-like organic matter can be effectively removed.Additionally,the study of its floc properties revealed that:fly ash-based PAFCS has the largest floc particle size,which gives it good settling properties;fly ash-based PAFCS has a high strength factor and a good network chain structure,which achieves deep treatment of coking wastewater through electro-neutralisation,adsorption and sweeping action.