Investigation On The Utilization Of Desulfurization Ash As Walling Material

At present, semidry flue gas desulfurization technology has been widely used in steelindustry in China. The resulting desulfurization ash is difficult to be utilized because of itscomplicated components and poor stability. Discarding of the desulfurization ash not onlyoccupies substantial amounts of land, which is high-cost, but also causes serious pollutionto the environment due to its harmful impurities. Therefore, exploration of an effectivemethod for treatment and utilization of desulfurization ash is of very important social andeconomic significance to the development of circular economy and the construction ofresource-saving society.The chemical compositions, crystal structure and morphology of the desulfurizationash, slag and samples after hydration were analyzed by the techniques of X-ray diffraction(XRD), scanning electronic microscope (SEM), and energy dispersive spectroscopy (EDS).The results show that the main phases in the semidry desulfurization ash are calciumsulfite, calcium sulfate, calcium hydroxide, etc, while those of slag are calcium aluminate,calcium silicate, etc. These ingredients react with each other to generate calcium silicatehydrate, hydrated calcium aluminate, ettringite, etc. SEM, EDS and XRD analyses revealthat the hydration products, containing the elements of Ca, O, Al, Si, etc, are rod-likecrystals, which consist of the phases of hydrated calcium silicate, hydrated calciumaluminate and ettringite.CaSO3is the main factor which affects the application of semidry desulfurization ash.Oxidation of CaSO3to CaSO4is a key technology to solve the problem of desulfurizationash application. The dissertation selects (NH4)2S2O8and H2O2as the oxidant to study howthe amount of the oxidizing agents, the categories of catalyst, reaction time, reactiontemperature and the amount of the additives affect the conversion rate of calcium sulfite.The results show that the optimum process, when using (NH4)2S2O8as oxidant, wasdetermined to be as follows: using CuSO4as catalyst, the molar ratio of CaSO3to(NH4)2S2O8to be20:1, adding0.2wt%(accounting for the weight ratio of thedesulfurization ash) of (NH4)2SO4, then reacting at a temperature of40C for3hoursunder stirring. More than50wt%of CaSO3can be oxidized under this process. Theoptimum process, when using H2O2as oxidant, was as follows: the molar ratio of CaSO3to H2O2to be20:3, adding2.510-5mol (in every hectogram desulfurization ash) CuSO4as catalyst, reacting at room temperature for3hours. More than60wt%of CaSO3can beoxidized under this process.The performance indicators required for the fabricated brick samples are reached byadopting the optimum oxidizing process, and using desulfurization ash and slag as starting material for wall brick fabrication, sodium citrate as the retarder, and water glass as theactivator. The results show that the optimum process, when using (NH4)2S2O8as oxidant,was determined to be as follows: using CuSO4as the catalyst, adding0.2wt%of sodiumcitrate and0.1wt%of (NH4)2S2O8.The brick samples fabicated by this program are low-intensity and easy to crack, furthermore, ammonia gas released in the stirring process. So itis not suitable for mass production. But the brick samples prepared by the program, whichuses H2O2as oxidant, are high-intensity and not easy to crack, and no harmful gas isreleased. The results show that the optimum process, when using H2O2as oxidant, wasdetermined to be as follows: adding H2O2with the molar ratio of CaSO3to H2O2of20:3,adding2.5×10﹣5mol (in every hectogram desulfurization ash) CuSO4as the catalyst,0.1%of sodium citrate as the retarder,0.4%of CaO as the activating agent. Thecompressive strength of the samples which are prepared under the program, have reachedthe standard of JC239-2001“fly ash bricks” M25brick.Considering that the brick samples formed by vibration need three days before moldstripping and the long production cycle, the pressing method was chosen to produce bricksamples. Selecting H2O2as the oxidant, the influence of molding pressure, curing time,activating agent species and dosage and slag particle size and dosage on the samplestrength were investigated. The results show that with the increase of molding pressure, atrend that first increase and then decrease appeared in the strength of the brick. Thestrength of the brick sample obviously increases when increasing the curing time,decreasing the slag size or increasing the slag proportion. The strength of the brick sampleincreases in a certain extent when different activating agents are used, however, theincrease is very limited. Using the standard sand as the skeletal material, the brick strengthfirstly increases, and then decreases with the increasing of skeletal material dosage.Summarizing the experimental results and economic factors, the optimum experimentalprogram was determined to be as follows: taking the same mass ratio of desulfurizationash and slag, adding0.8%of sodium silicate as the activating agent,0.1%of sodiumcitrate as the retarder,7%of standard sand as the skeletal material,18%of water, thenstirring for10minutes, compression molding under24MPa, curing in the condition of80℃and90%humidity for6h, then steam curing at100C for8h,and then curing innature condition for14days. The compressive strength and drying shrinkage values of thesamples, which were prepared following the program, have reached the standard ofJC239-2001“fly ash bricks” M20brick, but the frost resistance needs to be further studied.