Preparation And Properties Of Sol-bonded Magnesia-calcia Hot Gunning Mixes

During the clean steelmaking process,the different service environment of the refractory linings will cause localized damage to high-temperature furnaces.And those furnaces must be shutdown for maintenance in advance,which seriously hinders the steel production.Therefore,the periodical on-line hot repairement of the damaged zones by gunning mixes can ensure the efficient and stable production of clean steel.Magnesia-calcia hot gunning mixes are widely employed for steelmaking systems owing to their excellent corrosion resistance,thermodynamic stability and steel purification properties.Ane polyphosphates are the most often used chemical binders in magnesia-calcia hot gunning mixes.However,the existence of phosphorus element will increase the risk of molten steel phosphorization.Therefore,the development of phosphorus-free magnesia-calcia hot gunning mixes is of great significance for improving the quality of molten steel and ensuring the efficient production of clean steel.Based on the problems metioned above,the damage mechanism of ployphosphate-bonded magnesia-calcia hot gunning mixes and its phosphorization affection for clear steel were firstly studied in this thesis.Then the wollastonite sol bonded gunning mixes and the colloidal silica bonded gunning mixes were prepared.According to the simulation of the hot gunning process,the hot adhesion properties of the two mixes were investigated for the choice of binder used in the magnesia-calcia hot gunning mixes.After that,the viscosity change laws of gunning mixes during the gunning process as well as the effects of the addition of water and dispersants?FDN,citric acid,SD80?on the rheological characteristics of the matrix slurries were studied to improve the workability of colloidal silica bonded gunning mixes in virture of the rheological theroy.Moreover,the plasticizers?bentonite,carboxymethyl cellulose?and sintering aids?nano titanium dioxide,fume silica?were added into the colloidal silica bonded gunning mixes to enhance the hot adhension properties,of which the mechanism was also revealed.Finally,citric acid and?-methacryl oxypropyl trimethoxy silane?KH-570?were introduced into colloidal silica bonded gunning mixes to improve the intermediate temperature performance without damaging hot adhesion properties.Based on the above research,the main conclusions can be concluded as follow:?1?At relatively higher temperatures,Na₂O and P₂O₅ in the polyphosphate-bonded magnesia-calcia hot gunning mixes would be volatilized,resulting in a decrease in the binding phase-calcium magnesium sodium phosphate.Therefore,the gunning mixes have higher apparent porosity,lower cold modulus of rupture and lower cold crushing strength.Moreover,the microstructure of the polyphosphate-bonded magnesia-calcia hot gunning mixes is loose and porous.There exists a large gap between gunning mixes and magnesia birck,which increases the spalling risk of gunning mixes.In addition,using polyphosphate as binding agent in magnesia-calcia hot gunning mixes would increase phosphorus in molten steel.And the phosphorus content in molten steel gradually increases with the increase of polyphosphate content.?2?The rheological curves of the wollastonite sol show shear thickening,and they were similar to those of dilatant fluid.Since the gel structure formed by the hydrolysis of ethyl orthosilicate is similar to the structure of the pseudo wollastonite phase,the main formed phases of wollastonite sol are pseudo wollastonite after heat treatment at 800^oC,900^oC and 1000^oC,respectively.When the wollastonite sol is used as the binding agent of magnesia-calcia gunning mixes,the magnesia in gunning mixes could accelerate the formation of the three-dimensional network structure in the wollastonite sol,then strengthens the connection between the gunning mixes and the magnesia bricks.At the same time,wollastonite could react with the constituent of gunning mixes to form low-melting-point phases,which promotes the sintering between the gunning mixes and the magnesia bricks.Thus the hot adhesion performance of gunning mixes is improved.When 4wt%of the wollastonite sol is introduced into gunning mixes,the gunning mixes present low hot rebound loss rate,uniform gunned layer and optimal perporties.Compared with the polyphosphate-bonded gunning mixes,the one using the wollastonite sol as binding system has better hot adhesion properties and worse high-temperature mechanical properties due to the higher content of low-melting-point phases.?3?When the colloidal silica is added into magnesia-calcia gunning mixes as binding agent,the magnesia in gunning mixes could promote the dehydration and condensation of silanol groups in colloidal silica,which could accelerate the formation of network structure in the gunning mixes,thereby improving the hot adhesion properties of gunning mixes.The colloidal silica bonded gunning mixes with particle size distribution coefficient q of 0.28,high calcium magnesia-calcia sand?CaO:55wt%?as aggregates and5wt%of colloidal silica perform better mechanical properties and higher bulk density.At the same time,the hot rebound loss rate of gunning mixes is relatively lower,the gunned layer is more homogeneous and the bonding strength is higher.Compared with the polyphosphate-bonded gunning mixes,the one using colloidal silica as binding system performs better hot adhesion properties and high-temperature mechanical properties.And the hot adhesion properties are comparable with wollastonite sol bonded one.?4?The viscosity change laws of colloidal silica bonded magnesia-calcia gunning mixes during gunning process can be concluded as follows:The viscosity of matrix slurries of gunning mixes should be as low as possible to ensure that the particles could be smoothly embedded in the gunned layer and reduce the distance between the particles as much as possible.In the other hand,the apparent viscosity of the whole gunning mixes should be as high as possible to ensure the stability of the gunned layer.The rheological curves of the matrix slurries of colloidal silica bonded gunning mixes present pseudoplastic fluid characteristics,and the suitable water addition is about 22wt%.The addition of FDN,citric acid,and SD80 in slurries conributes to improving the rheological properties,and the optimal addition are 0.3wt%,0.2wt%and 0.1wt%,respectively.Compared with the addition of FDN and citric acid,the addition of SD80 in slurries presents plastic performance,which would weaken the impact between particles more effectively,thus reducing the risk of rebound and lamination.In a word,the gunning mixes with 0.1wt%of SD80 performe best rheological properties.?5?The addtion of bentonite and carboxymethyl cellulose can improve the thixotropy of colloidal silica bonded magnesia-calcia gunning mixes,leading to the kinetic energy of the gunning mixes absorbed effectively during spraying,thereby reducing the hot rebound loss rate of the gunning mixes.When 1wt%of bentonite and0.1wt%of carboxymethyl cellulose are added together,the gunning mixes get a better combination property.The introduction of nano titanium dioxide further reduces the hot rebound loss rate of the colloidal silica bonded magnesia-calcia gunning mixes.Because the addition of nano titanium dioxide could form new phases,and changs the composition and distribution of the phase at the grain boundary,thus promoting the sintering of the gunning mixes.When 2.0wt%of nano titanium dioxide and 2.0wt%of fume silica are added,the performance of the gunning mixes is optimal.?6?The addition of citric acid and KH-570 both can enhance the intermediate temperature performance of colloidal silica bonded magnesia-calcia gunning mixes.For citric acid-containing ones,the carboxylic groups in citric acid and magnesia particles would form a stable Mg-O-C monodentate ligand,inhibiting the hydration of magnesia particles.Therefore,the intermediate temperature performance of specimens enhanced.For KH-570-containing ones,the organic-inorganic hybrid interpenetrating network could be formed in the gunning mixes by virtue of the"dispersion"of the organic end of KH-570 as well as the reaction between the hydrolysate of KH-570 and colloidal silica.Though the organic part of the network in gunning mixes would be volatilized after the high temperature heat-treatment,the residual silicon still maintaines the original network structure,and achieves excellent intermediate temperature performance.