Nonhydraulic Bonding Mechanism Of Cold-bonded BOF Dust Pellets And Agglomeration In Cleaning System

Basic oxygen furnace(BOF)dust is a kind of solid waste collected by dry flue gas cleaning system in metallurgical steelmaking process.Because BOF dust contains more than 50%total iron(TFe)and a certain amount of over-burned MgO and Ca O,it has a high secondary utilization value.It is the most economical and reasonable way to use BOF dust with binder to prepare pellets,and return pellets to BOF at room temperature(cold-bonded)to realize"internal circulation"of BOF to dust cleaning system to BOF.Nevertheless,there are two problems in the BOF dust internal circulation process:first,the currently used binders are difficult to inhibit the digestion and expansion of over-burned MgO and Ca O in BOF,which affects the pellet strength;second,the internal circulation is bound to aggravate Zn-containing dust in the cleaning system to form high-strength agglomerates,which hinders the normal operation of cleaning system.Consequently,it is necessary to investigate the problem of cold-bonded pellet strength and cleaning system agglomeration,in order to provide theoretical and technical guidance for the internal circulation of BOF dust and the sustainable development of steel enterprises.In order to solve the problem of pellet strength,based on the nonhydraulic cementitious properties of MgO in BOF dust,Mg Cl₂or KH₂PO₄was added to prepare magnesium oxychloride(MOC)and magnesium potassium phosphate(MKP)cementitious binders to improve the pellet strength.The strength and bonding mechanism of MOC and MKP binders in pellets were investigated,via analyzing the hydration and crystallization behavior of MOC binder prepared by over-burned MgO at1500?,and the raw material ratios of MKP binder.Based on the strength and structural characteristics of the MOC and MKP binders in the pellets,the differences in the reaction rate and bonding mechanism of the two binders in the pellets are analyzed.Furthermore,in order to overcome the problem of cleaning system agglomeration,the methods to control the high-strength Zn-containing agglomerates in the cleaning system were proposed by analyzing the agglomeration formation mechanism.The main research contents are summarized as follows:(1)The bonding mechanism of MOC binder in pellets was clarified,via investigating the hydration and crystallization behavior of MOC binder prepared by over-burned MgO at 1500?.The results show that the over-burned MgO prepared at1500?and Mg Cl₂can undergo hydration reaction,forming adhesive gels to absorb BOF dust particles.The gels develop to 5Mg(OH)₂·Mg Cl₂·8H₂O crystalline phase(5phase)with a reticular structure after solidification,and the crystalline phase produces mechanical strength of the pellets by wrapping and connecting bridge.The hydration reaction produced by over-burned MgO is incomplete,and the hydration reaction degree is 52-63%.Under industrial conditions,if the pellets satisfy the strength requirement exceeding 6 MPa within 48 hours,MgO content in the BOF dust is at least 4%.(2)The research on the ratio and bonding mechanism of MKP cementitious binder in cold-bonded pellets found that,MKP binder can form K-Struvite crystals and make the pellets have higher mechanical strength under magnesium-to-phosphate molar ratio of 4 and water-to-binder mass ratio of 0.5.By adding 2%MKP binder,the pellets can reach 6.9 MPa within 48 hours,and P-containing substances will not be decomposed under the BOF high temperature conditions.The bonding mechanism of MKP binder in pellets is divided into two types:in the binder concentrated areas,the massive and dense K-Struvite crystals wrap BOF dust particles;in the binder decentralized areas,the fine K-Struvite crystals are filled between the dust particles or adhere to the surface of dust matrix,connecting the dust particles into a whole.(3)Comparing the reaction rate and bonding mechanism of MOC and MKP cementitious binders in the pellets,it is concluded that MKP binder has a faster initial reaction rate,which is beneficial to improve the early pellet strength,while the pellet strength containing MOC binder increases quicker in the later curing process,which is more conducive to improve the later pellet strength.The acicular 5 phase crystals formed by MOC binder have strong surface polarity and higher bonding strength.They are interlocked to generate a network structure,which forms a unified whole with high strength after the dust particles are wrapped.The prismatic K-Struvite crystals formed by MKP binder have strong intermolecular force and are densely packed with each other.The strength of MKP binder is higher,and its strength is about 1.2 times that of MOC binder after 7 days of curing.They are filled in the gaps between dust particles and form solid bridges to improve the pellet strength.(4)Analyzing the formation mechanism of high-strength agglomerates in the evaporative cooler of BOF dry flue gas cleaning system,it is found that,under high temperature conditions,Zn O and Fe₂O₃in BOF dust chemically bond through a solid-phase reaction to form a dense structure of Zn Fe₂O₄,which is the the primary cause of dust agglomeration strengthening in evaporative cooler.Industrial practice results show that,decreasing the inlet temperature of evaporative cooler to less than900?and controlling the content of low melting point substances containing Na and K to less than 0.5%are effective measures to reduce dust agglomeration and and ensure the"internal circulation"of BOF dust.