Study On The Oxidation Of Semi-dried Desulfurization Ash And Its Applications

The semi-dried desulfurization ash in the steel industry is very difficult to utilize due to its instability and complicated compositions.The mountainous desulfurization ash occupies a large area of land.Furthermore,the harmful impurities in the ash cause more serious environmental pollution.Therefore,it is of great significance to find an effective method to treat desulfurization ash.In this thesis,the oxidation process of semi-dried flue gas desulfurization ash was studied.First,various experimental conditions such as the oxidant,catalyst,oxidation temperature and oxidation time of desulfurization ash were explored.Then,its two possible application approaches were explored as the raw materials for the production of oxidized desulfurization ash/slag sintering-free bricks,and the cementing materials composed of oxidized desulfurization ash-steel slag-slag.Also,the influencing factors of the performance of the prepared sintering-free bricks and cementing materials were studied.The main contents and results are as follows:The first part concerns study on the oxidation of desulfurization ash.The CaSO₃component in semi-dried flue gas desulfurization ash was oxidized by?NH₄?₂S₂O₈ and H₂O₂.The results show that the oxidation rate of?NH₄?₂S₂O₈ to CaSO₃ is higher than that of H₂O₂.However,ammonia gas is released during the stirring process when?NH₄?₂S₂O₈acts as the oxidizer,which is not suitable for bulk use.CuSO₄,NiSO₄,Fe?NO₃?₃,MnSO₄were utilized as catalysts for H₂O₂ oxidation of CaSO₃,respectively,and the catalytic reaction effect from strong to weak is ranked as CuSO₄>NiSO₄>MnSO₄>Fe?NO₃?₃.By changing the amount of H₂O₂ and CuSO₄,reaction temperature,oxidation time,and suitable oxidation conditions were obtained.The oxidation conditions were determined to be as follows:the molar ratio of CaSO₃ and H₂O₂ of 20:1,the concentration of CuSO₄ of2.5×10^-55 mol per 100 g desulfurization ash,the reaction temperature of 25°C,and the reaction duration of 1 h.As a result,the oxidation rate of CaSO₃ reaches 51.27 wt.%,which meets the requirements of large-scale applications of desulfurization ash.The second part involves the preparation of unburned bricks from oxidized desulfurization ash and slag.The effects of the preparation conditions,such as the activator types,its content,aggregates and curing methods,on the performance of oxidized desulfurization ash and slag were studied,using oxidized desulfurization ash and slag as the raw materials.The results show that the order of the excitation effect from strong to weak is ranked as:CaO>sodium citrate>sodium silicate>KAl?SO₄?₂·12H₂O>Na₂SiO₃·9H₂O>CaCl₂>NaOH.Composite activator is more effective than single activator.The pressure-resistance strength of the test block increases first and then decreases with increasing standard sand?aggregate?content.When the amount of the added standard sand is 5 wt.%,the average pressure-ressitance strength of the test block reaches the maximum.Through a series of experiments,the optimized compositions of the test block is as follows:the slag micropowder and the oxidative desulfurization ash at a mass ratio of 1:1,2 wt.%CaO,0.5 wt.%sodium silicate and 0.1 wt.%sodium citrate as the composite activator,5wt.%standard sand as the aggregate.The pressure-resistance strength of the test block prepared reaches 25.95 MPa at 100°C steam curing condition for 8 h.Standard bricks with the optimized compositions were prepared under the conditions of steam curing at 178°C in 10 atm for 8 h,then naturally curing for 7 days,and their pressure-resistance strength reaches 14.76 MPa.When steam curing is at 100°C in 1 atm for 8 h,and then natural curing for 7days,their pressure-resistance strength reaches 30.65 MPa.It is in line with the MU30 strength grade standard in JC239-2014?Autoclaved Fly Ash Brick?.Frost resistance and shrinkage rate are in line with the technical indicators in JC239-2014?autoclaved fly ash brick?.Finally,the preparation of cementing materials was studied,using oxidized desulfurization ash,slag micropowder and steel slag as the raw materials.Using the main components of Ca?OH?₂ and CaSO₄·2H₂O in the desulfurization ash as the initiator,the effects of Ca?OH?₂,CaSO4·2H₂O and their composites on the volumetric stability and mechanical properties of steel slag-slag cementing materials were investigated.The results show that when the content of CaSO₄·2H₂O is more than 12 wt.%,the volumetric stability of the test block is good,and the pressure-resistance strength also increases.When the content of CaSO₄·2H₂O is 16 wt.%,the average pressure-resistance strength is 16.46 MPa.When the CaSO₄·2H₂O content continuously increases,the average pressure-resistance strength of the test block decreases.When the content of Ca?OH?₂ is more than 1 wt.%,the steel slag-slag cementing material shows good volumetric stability.With the Ca?OH?₂content increasing,the pressure-resistance strength also increases.When the content of Ca?OH?₂ exceeds 8 wt.%,the increase of pressure-resistance strength is not obvious.Through orthogonal and single factor optimization experiments,the influence of three factors on the pressure-resistance strength of steel slag-slag cementing material from strong to weak is ranked as:CaSO₄·2H₂O>Ca?OH?₂>steel slag micropowder.The optimal proportion is obtained as follows:40 wt.%steel slag micropowder,4 wt.%calcium hydroxide,4 wt.%calcium sulfate dihydrate and 52 wt.%slag.Its pressure-resistance strength reaches 16.84 MPa after the steam curing.The pressure-resistance strength can be effectively improved by adding Ca?OH?₂ and CaSO₄·2H₂O to the cementing material composed of oxidized desulfurization ash,steel slag and slag,and the amount of oxidized desulfurization ash also is increased.When adding 1 wt.%Ca?OH?₂,1 wt.%CaSO₄·2H₂O,38 wt.%slag powder,40 wt.%steel slag powder,20 wt.%oxidative desulfurization ash,the average pressure-resistance strength of the cementing material reaches 16.53 MPa.