Improvement of parameters for production of refractory composites based on zirconia

Refractory materials based on zirconia are used in iron-steel industry particularly thanks to enhanced corrosion resistance. The metering nozzles used in Continous Casting Machine (CCM) are produced with raw materials based on zirconia. New generation CCM sliding gate system needs MgO stabilized zirconia nozzles with low porosity. In spite this refractory ceramic has a highest corrosion resistance in comparison to all available refractory materials at the interphase steel/slag/ceramic suffers under thermal shock attack. In this study the parameters for the manufacture of refractory based on zirconia were studied and then alternative composite body enriched with alumina was synthesied. The effect of alumina addition and spinel formation on thermo-mechanical properties were discussed. Zirconia metering nozzles used during continous casting of steel were also characterized by several tests to make comparison with the results obtained during the experiments. Density which was the most indicative property of nozzle quality changes between 4,60-5.40 g/cm3. Two types of raw materials based on zirconia were studied to synthesize refractory material for metallurgical applications. MgO stabilised fused zirconia and chemicaly precipitated monoclinic zirconia were supplied and characterized using EDXRF, XRD, SEM, gas pycnometer, laser particle sizer. MgO stabilised fused zirconia powders were subjected to various tests to determine the suitable powder preparation method. Several mixtures with different coarse/fine ratio were mixed in dry form. Particle size distribution is checked according to Dinger&Funk theory. Coarse/fine ratio was selected as 60:40. 40x40x10mm samples were compacted by laboratory press with 255MPa and sintered in air atmosphere. Density and porosity were tested with water boiling method and the results were compared. Density for Mixture E after dry mixing tests was found to be 4,27 g/cm3 as a highest result. Mixture E was selected for the further tests. The flow properties were studied with Hall Funnel. The powders with grain size finer than -150 micron did not show free-flow property which is needed to achieve uniform structure after compaction. The effect of granulation on flow and physical properties of ceramic body was studied with further tests. Mixture E was mixed in drum type mixer with addition of liquid binder. A commercial type of PVA preparation was selected as a binder. The quantity of binder was changed between 3% to 5% by weight. The soft and ball shaped granules were dried and screened. +500 micron, +250 micron, +125 micron were collected and compressed into pellets with 28 tonnes. The samples with dimensions of 40x40x10mm were sintered at different temperatures for 5 hours. The results were compared with previous results for dry mixed Mixture E. Mixture E showed good flow properties after granulation but the granules were soft and the shape was not regular. The density increased to 4,80 g/cm3 after granulation with drum type mixer. Sintering temperature was selected as 1700oC and decided to be kept same for further tests. In the next stage process with spray drying was investigated through careful literature review. Buchi Laboratory Type Spray Dryer was used during the granulation tests. The solid composition based on MgO stabilised fused zirconia was prepared with -300 micron and 3-5 micron. Coarse/fine ratio was 60:40. Wet milling in the zirconia cup with zircon balls decreased the grain size of powder. Change in grain size with milling time was tested. Milling time was optimised as 4 hours. Grain size distribution of powder was measured with laser particle sizer. D50 of milled powder was found to be 2,78 micron. The suspension for spray drying was prepared after wet milling. PVA and ammonium salt of polyacrylic acid were selected as a binder and dispersant respectively. A very strict mixing procedure was carried out. pH and zeta potential of suspension were measured. The solid/liquid ratio and binder content were determined according to resulting granule properties. The structure and flow properties of granules were characterized with laser particle sizer, tapping and flow tests. The granulation increased with increasing binder content and solid content. Processing conditions for spray drying like pumping ratio, air flow rate, temperature were clarified accoding to granule properties. d50: 7,5 micron and d90: 28,9 micron were measured for the resulting granules. "Angle of repose" was calculated with "Hall Funnel" and powder flow was classified as "good". The granule shape was identified by using SEM. Poured density was found to be 1,16 g/cm3. These powders were then uniaxially compressed into sample shape with sizes 50x10x10mm with a pressure of 255 MPa. The resulting pellets were subjected to sintering in air at 1700oC for 5 hours. Density was achieved as 5,40 g/cm3 and open porosity was found to be 0,16%. Hardness were measured to be 1237 kg/mm2 by Vickers indentation test. The parameters for the spray dry processing were decided to be kept same during the tests with monoclinic zirconia. The phase stabilization of zirconia was studied during the tests in the next stage.The process conditions for in-situ stabilization during sintering was investigated. Sintered magnesia from Kütahya Manyezit İşletmeleri A.Ş. (KÜMAŞ A.Ş.) was used as a stabilising agent. The compositions based on chemically precipitated monoclinic zirconia were prepared with a MgO content between 7 and 11% mol. XRD examination was made to understand phase transformation according to MgO content. The samples were pressed after wet milling and sintered at 1700oC for 5 hours. The hardness test was also applied after sintering in order to determine change in hardness with phase distribution. 9% mol MgO was found to be optimum to achieve similar phase distribution with commercial product. The microstructural features and the phase composition were examined using SEM and XRD. Sintered cubic crystals with size of 25-30 microns were produced. Surrounding and covering fine monoclinic and tetragonal crystals were identified. The powder based on monoclinic zirconia with a size of 15 micron and dead burnt sintered magnesia (9% mol) with a size of -63 micron was mixed by wet milling for 4 hours as applied during the tests with MgO stabilized zirconia raw materials. D50 of milled powder was measured to be 2,83 micron with laser particle sizer. The suspension for spray drying was prepared as before carefully. The bulk density of granules was measured as 1,40 g/cm3. The samples were shaped with granules based on monoclinic zirconia with addition of MgO. Sintering temperature was kept same as before but the procedure was changed. Ageing stage at 1100oC for 4 hours was applied during cooling for transformation of tetragonal to monoclinic. The density after sintering including stabilising process was measured to be 5,24 g/cm3. Bending strength was measured with three point bending test. Abrasion resistance was measured with alumina balls. The results were compared with the data for samples based on PSZ with MgO. Both of the results were lower than that of samples prepared with MgO stabilized zirconia. Thermal expansion behaviour was similar with commercial product that was showing expansion at 800-900oC during cooling due to tetragonal-monoclinic transformation. In the last stage the effect of reactive alumina addition on the thermo-mechanical properties of monoclinic zirconia during the sintering and in-situ stabilization with dead burnt magnesia was studied. The composition of monoclinic zirconia and MgO was prepared with 5, 10, 15% alumina addition by weight respectively. MgO content was kept unchanged. The powder with PVA as a binder was mixed and reduced in size by wet milling. Milling procedure was kept same in order not to change grain size then the powder was shaped by press with the same procedure as before. The samples were then sintered at 1700oC with the same procedure applied before for in-situ stabilization. After sintering; - Density and apparent porosity - Hardness - Optical microscope - SEM (with EDS analysis) and mapping - XRD - Bending strength - Thermal shock resistance - Abrasion resistance As a conclusion; Granulation improved flow properties of powder to achieve uniform filling resulting with high density ceramic structure after sintering. All physical and mechanical properties improved after granulation. The process with Buchi Laboratory Spray Dryer was optimised with %60 solid content. Binder ratio was determined as 5% by wt. Two group of samples were tested. First group was based on MgO stabilized zirconia. Second group samples were prepared with monoclinic zirconia with stablising agent (sintered MgO). The density of samples shaped with press after granulation with spray dryer was found 5,40 g/cm3 and 5,24 g/cm3 for first group and second group respectively. The hardness of samples based MgO-PSZ was higher than that of based on monoclinic zirconia with sintered magnesia. MgO.Al2O3 spinel phase formed during sintering at 1700oC with addition of alumina. The content of spinel phase increased with increasing alumina ratio by weight. Monoclinic zirconia and spinel with small crystal size increased after sintering with increasing alumina content. All physical and mechanical values due to addition of alumina decreased in comparison to reference sample without alumina The bending strength increased after thermal shock cycles with alumina addition. Thermal expansion increased due to spinel formation during sintering.